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Discussion Starter · #1 · (Edited)
Thanks in advance for any advice.

Short version: When the engine heats up, both front brakes apply and continue to drag until the engine temperature comes down.

NOTE: Updated 11/9/2020 - After some additional testing, this is not a 'heat' problem. The brakes apply intermittently but release after sitting for awhile.

Short version: Periodically, all four (disc) brakes will apply. Sometimes so much, that I have to floor the accelerator to get the truck to move.

Long version: I bought a '00 4WD (in March) with ~200K miles, replaced the short block and had the transmission rebuilt. After driving it for a couple of months (I don't put many miles on it), I was waiting in line in a drive-thru, on a 90 degree day, and when I released the brakes to pull forward, the truck would not move (I glanced at the gauges, and the engine was running ~200 degrees). I had to give it some accelerator to get it to move forward. After completing my transaction, the brakes remained applied the entire time getting back to my house (5 minutes away - I had to stay on the accelerator the whole time to get it t move).

In subsequent trips, I began to notice that whenever the engine got a little above operating temperature (195 degrees), the brakes would start to drag (while I'm driving, not necessarily from a stop). The brake pedal would be high, and if I let up on the gas pedal, the truck would lurch forward (in a straight line) and stop entirely. Once the truck cools down though, after the truck sits, the brakes release. It would get so bad sometimes, that I would have to pull over and let the truck cool down. At no time, was the engine overheating. Generally, the engine runs pretty cool.

The couple of times that its happened, and I've been able to do some checking, I observed that the both front all four wheels won't spin. I checked the front and both front hubs are very hot. I checked the brake reservoir, and the fluid is between the marks. On the lower end in fact. I've also noticed, that if I drive very conservatively, and the engine temperature stays below 200 degrees, I don't have 'as much' of a problem (I can sometimes feel the brakes start to apply, but not entirely.)

Those symptoms, combined with the 'straight line lurching', has led me to conclude that the front brakes are 'self' applying. Because it appears to be both front all four wheels at the same time, I've eliminated the possibilities of it being degraded brake lines, caliper slides (which, I pulled and lubed any way) or calipers. It seems to be something common to both front wheels and brought on by heat.


My diagnosis: The fact that the brake lines running from the master cylinder to the ABS unit run pretty close to the D/S exhaust manifold, (and some research) leads me to think that maybe the brake fluid is expanding (boiling?). But, then I would expect it to push back into the master cylinder (where there is room). The other possibility I considered was a bad master cylinder (clogged return hole?). But then, I'm not clear on how that would be affected by engine heat.

My prognosis: Put some heat shield around the brake lines, flush the brake system and upgrade the brake fluid (DOT 4? - maybe not necessary, if I just replace the degraded fluid). Replace the brake master cylinder.

Has anyone experienced these symptoms? And, most importantly, has anyone solved it?
 

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You're on the right track that the fluid should expand into the reservoir. It's possible that the piston in the master cylinder is slightly depressed (booster pushrod too long or pedal not returning completely?), so that it covers the return hole and won't allow fluid to return. Next time it happens, try loosening the two nuts holding the master cylinder to see if the pressure releases. That will tell you.

The fix would be a new master cylinder, new booster, or pushrod adjustment (if it's adjustable). If the pedal is not coming up all the way, you'll need to figure out why.

I've seen this problem many, many times on motorcycles when people install aftermarket levers that don't fit quite right. I even witnessed a crash caused by it.
 

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^^^^^^^^^^^^^^^ THIS ^^^^^^^^^^^^^^^

As a quick test you can space the MC out from the booster with some washers.
 

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Mr Goodwrench's Evil Twin
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I would first make an effort to determine that engine heat, rather than brake heat, is causing the issue. To do this you could start the truck when it's been sitting long enough that everything is cool (like in the morning). Run the engine long enough to fully warm it up without driving the truck. A piece of cardboard in front of the radiator and holding the RPMs at 1500-2000 should get it nice and warm if it otherwise doesn't want to fully warm up. Once it is fully warmed up, check to see if your brakes are dragging. If they aren't, then brake heat is causing it, not engine heat.

Boiling brake fluid has the same affect on brakes as air in the brakes... a spongy brake pedal and loss of braking ability. Brake fluid degrades as a result of moisture, which lowers the boiling point of brake fluid. If your brake fluid is boiling, you'd be experiencing reduced braking ability.

Any issue with the master cylinder as described above would result in an issue that is common to both the front and rear brakes, which is certainly something to check for. If it truly is JUST the front brakes, the master cylinder is not likely to be the cause. If the problem is on both front and rear brakes, the master would be pretty high on my suspect list.

Keep in mind that metal expands with heat, so even if your calipers are free and well lubricated when cool, doesn't mean they are free when hot. If there is any amount of buildup on the slide pins, they could be binding when they get warm and expand.
 

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Discussion Starter · #6 ·
On @thebadwrench advice, I drove the truck aggressively over the, relatively warm, weekend, but I was not able to reproduce the issue (and, not able to get the engine temp above 198 degrees - good news as far as that's concerned I suppose).

Then, driving it around today, and not nursing it, the brakes started to apply (not full lock-up, but it started to require a little more accelerator to maintain speed). The engine temperature remained below 195 degrees. I got it home while it was still happening, jacked it up and all four wheels were locked (at least, could not be spun by hand).

This leads me to conclude that I have an issue with the brake master cylinder or the booster. What I forgot to do (as TomA suggested), was to loosen the BMC bolts to see if the problem went away.

The next time it happens, I'll loosen the bolts and see whether we're talking BMC replacement or adjustment (my money is on BMC replacement).
 

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Mr Goodwrench's Evil Twin
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Before you go replacing the master, just unbolt it and make sure the pushrod isn't sitting crooked. When it happens, have you tried lifting the brake pedal with your foot?
 

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Discussion Starter · #8 ·
I have not tried lifting the brake pedal, because it is always sitting at the top of the travel when this occurs. It has the feeling of when you 'pump up' the brakes, and hold the pedal, when bleeding the system.

I will unbolt the master. But, it makes no sense to me how the pushrod would become 'uncrooked' after sitting awhile.

@TomA suggested that maybe the return hole is slightly covered, but over time releases the pressure. That makes sense to me.
 

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B4U Task Force Admin
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I have not tried lifting the brake pedal, because it is always sitting at the top of the travel when this occurs. It has the feeling of when you 'pump up' the brakes, and hold the pedal, when bleeding the system.

I will unbolt the master. But, it makes no sense to me how the pushrod would become 'uncrooked' after sitting awhile.

@TomA suggested that maybe the return hole is slightly covered, but over time releases the pressure. That makes sense to me.
I tried an aftermarket booster that would clear my fuel rail covers. It was doing the same thing. The peddle got to the point that there was no play when I stepped on the brake. Not sure why, but I tried lifting the peddle...it worked.
 

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94 4x4,01 Blazer
2001 Blazer 4dr 4wd LT
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before changing out any parts you can do a COMPLETE brake fluid change by bleeding each wheel until you get clear fresh fluid- just keep filling master with NEW fluid. it sounds like you have contamination(water) somewhere.Moisture does happen .
 

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Mr Goodwrench's Evil Twin
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I will unbolt the master. But, it makes no sense to me how the pushrod would become 'uncrooked' after sitting awhile.
Stranger things have happened
@TomA suggested that maybe the return hole is slightly covered, but over time releases the pressure. That makes sense to me.
This is why I suggested you check to see if your issue is specific to the front brakes or common to all 4 brakes. Sounds like you're on the right track
 

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Discussion Starter · #12 ·
Happened again today.

I got it home, and when I put it in drive, on level ground, with my foot off of the brake pedal, the truck did not move at idle.

I put it in park, left it running, loosened the BMC from the booster (about 1/16" - 1/8"), put the truck in drive and it rolled at idle.

I then put it back in park, tightened the master cylinder back up, put it in drive and it still rolled easily. My guess is that the brake fluid return hole is not opening fully.

This is a relatively new problem. I've owned the truck since March, and it first started happening in July. I don't drive it much, so maybe just lucky(?). It does look like a lot of brake work has been done (the front pads, rotors, calipers and the BMC are pretty clean). Which makes me suspect that maybe the previous owner was having this issue and threw parts at it. All of the parts are OEM. As such, I'm cynical that it is an adjustment issue.

Do I replace the BMC for good order? Or, should I be looking at something else?
 

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Your "safest" option, IMO, is to replace the booster and master cylinder with a new assembly, so presumably they're factory-adjusted. If you replace only the MC, pay attention when you install it and make sure the piston is not being pushed in. Before doing any of that, though, make sure the pedal is coming all the way up. You could have a wiring harness or something stuck in a position to prevent it, or some other problem.
 

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Discussion Starter · #14 ·
Before doing any of that, though, make sure the pedal is coming all the way up. You could have a wiring harness or something stuck in a position to prevent it, or some other problem.
I will check the brake pedal. If no issue there, I will take your advice and replace both components. I'll update this thread accordingly.

Thanks @TomA , @thebadwrench and @Rhotpursuit.
 

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Discussion Starter · #15 ·
When it happened today (a little - sitting on level ground, in gear, didn't move at idle), I tried pulling up on the brake pedal. It did not get better.

I'm going to order a booster/cylinder and I'll let you all know how it goes.
 

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Thanks in advance for any advice.

Short version: When the engine heats up, both front brakes apply and continue to drag until the engine temperature comes down.

NOTE: Updated 11/9/2020 - After some additional testing, this is not a 'heat' problem. The brakes apply intermittently but release after sitting for awhile.

Short version: Periodically, all four (disc) brakes will apply. Sometimes so much, that I have to floor the accelerator to get the truck to move.

Long version: I bought a '00 4WD (in March) with ~200K miles, replaced the short block and had the transmission rebuilt. After driving it for a couple of months (I don't put many miles on it), I was waiting in line in a drive-thru, on a 90 degree day, and when I released the brakes to pull forward, the truck would not move (I glanced at the gauges, and the engine was running ~200 degrees). I had to give it some accelerator to get it to move forward. After completing my transaction, the brakes remained applied the entire time getting back to my house (5 minutes away - I had to stay on the accelerator the whole time to get it t move).

In subsequent trips, I began to notice that whenever the engine got a little above operating temperature (195 degrees), the brakes would start to drag (while I'm driving, not necessarily from a stop). The brake pedal would be high, and if I let up on the gas pedal, the truck would lurch forward (in a straight line) and stop entirely. Once the truck cools down though, after the truck sits, the brakes release. It would get so bad sometimes, that I would have to pull over and let the truck cool down. At no time, was the engine overheating. Generally, the engine runs pretty cool.

The couple of times that its happened, and I've been able to do some checking, I observed that the both front all four wheels won't spin. I checked the front and both front hubs are very hot. I checked the brake reservoir, and the fluid is between the marks. On the lower end in fact. I've also noticed, that if I drive very conservatively, and the engine temperature stays below 200 degrees, I don't have 'as much' of a problem (I can sometimes feel the brakes start to apply, but not entirely.)

Those symptoms, combined with the 'straight line lurching', has led me to conclude that the front brakes are 'self' applying. Because it appears to be both front all four wheels at the same time, I've eliminated the possibilities of it being degraded brake lines, caliper slides (which, I pulled and lubed any way) or calipers. It seems to be something common to both front wheels and brought on by heat.


My diagnosis: The fact that the brake lines running from the master cylinder to the ABS unit run pretty close to the D/S exhaust manifold, (and some research) leads me to think that maybe the brake fluid is expanding (boiling?). But, then I would expect it to push back into the master cylinder (where there is room). The other possibility I considered was a bad master cylinder (clogged return hole?). But then, I'm not clear on how that would be affected by engine heat.

My prognosis: Put some heat shield around the brake lines, flush the brake system and upgrade the brake fluid (DOT 4? - maybe not necessary, if I just replace the degraded fluid). Replace the brake master cylinder.

Has anyone experienced these symptoms? And, most importantly, has anyone solved it?
I'll tell you what I had happened to me one time and it like to drove me crazy until I figured it out period one time I was in Houston my wife and I were down there in our Honda Accord and all the sudden the brakes were doing exactly what you're describing. I'm 200 mi from home I've got $25 in my pocket and I'm thinking the world is about to fall on my head period so I pull into the nearest Honda dealership because the car was under warranty tossing the keys tell him fix the problem they come back around with the vehicle in about 5 minutes and toss me the keys back say there you go you're good to go.well I've turned the wrench for a living for a lot of years so I was a little perturbed and then they told me that it was the floor mat was touching the brake pedal and applying the brakes just a little bit period start there but if it's not the floor mat as in my case and it sounds like you're coming down with a bad case of hydraulic lock from the temperature of the engine period sounds like you're on the right trail with the heat shield. Also check your master cylinder make sure that you're piston isn't sticking down sounds like it has some bleed by or something it could possibly be going out make sure you're also not leaking vacuum right at the brake booster sometimes that'll do that also. Good luck
 

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FLORIDA BUTCH
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There are different brake boosters that go on, even in the same year of truck. When I went to the Auto Store, I had 5 to choose from. We tried about 7 of them and none of them were right. They would look like they fit, but the pedal barley engaged. Then I seen my original booster # also had SVT on it. Some aftermarket boosters have the letters and some do not. When we put a new SVT in, everything fit and worked. So even thought the Auto Parts store had a complete booster and master set for a 1988 S10, on their shelf for all those years, it wasn't the right one for my 1988 Pickup.
 

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Discussion Starter · #18 ·
There are different brake boosters that go on, even in the same year of truck.
Great tip @FLORIDA BUTCH. That is a fact. I started looking for boosters/cylinders yesterday. There is a whole list for my application. I closed the computer and decided I'd wait until I can get under the hood and look for some part numbers or identifiers.
 

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Discussion Starter · #19 ·
Fixed! The problem was the brake master cylinder (my theory is that the return spring was shot/busted and the return holes weren't fully opening).

After looking at the job, and realizing that I'd have to crawl under the dashboard and remove and re-thread a bolt hiding behind the steering column, I handed it off it to my local shop.

Now that its fixed, I realize that I had been driving, with the brakes partially applied (either a little or a lot) since I got the truck 9 months ago (I don't drive it much).

Thanks everyone for the advice.
 
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