Power Brakes Gradually Locking Up

[dchwilson]

Hi Folks! New here. Seems like a really informative site. I was referred nere by another member of a FB group I'm part of called "Found on the Street." Maybe y'all can help me with a problem my mechanic and I have been struggling with.

The brakes on my 1976 GMC Sierra Classic keep locking up, gradually, while driving. It has power brakes; discs on the front and drums on the rear. I have not attempted to see if the brakes would lock up with just the engine on, but not moving, though I have not tried that. I don't think they would. I think it is all four wheels locking up instead of just the front or rear, but I only think that because all four hubcaps are really hot after it happens.

I've replaced the master cylinder, the booster, all brake pads, rubber brake hoses, calipers, wheel cylinders, and even the proportioning valve to no avail. When I bought the truck the master cylinder had been replaced with a rebuilt one, and I'm on a second brand new one just to see if that had something to do with it (and because they're inexpensive). The rotors are good and the drums are too (not scored). The parking brake works great and the wheel cylinders are not leaking.

I am at a loss. I do not know what else to do. I can't afford to keep throwing parts at it in hopes that fixes the problem. At this point the brakes have been flushed so many times that I cannot imagine trash in the lines could be the culprit. My mechanic adjusted them again today and told me if that didn't fix it he didn't know what else to do. Does anyone have any suggestions? Thank y'all in advance for any advice you can provide!

 

[Georgeb]

Hello and welcome. Sounds like you have tried really hard to correct this! My first thought after reading through your post would be to check if for some reason your pedal is not returning completely or there is an issue causing the brakes to be applied slightly by the brake push rod from the pedal to the booster. Do you feel the brakes dragging while driving? It may help to explain any other symptoms as the issue arises i.e. "Drives fine a first and gradually increases in brakes dragging as I drive further or the wheels spin freely with the engine off but increase in resistance once the engine is started."
The more info the better.

 

[dchwilson]

Reply

Hi George! Thank you for replying. I sure have been trying to get this to work properly; my truck is my daily driver now, and I think if I could get this fixed it would have many more years of service.

The pedal returns properly each time I apply the brakes, whether the truck is moving or not. However, as the brakes begin to lock up I cannot press the pedal very much to fully apply the brakes (so it doesn't have as far to return).

Out of desperation I visited NTB today because my usual mechanic doesn't work on Saturdays and because the franchise in my area is pretty decent. They were able to confirm that it's only the front brakes locking up. Their suggestion was to replace the master cylinder (again!) When I bought the truck it had a rebuilt (very recently installed) master cylinder on it, and since then I have installed two brand new ones as well. Apparently the service manager at NTB had an old Cutlass with a similar brake system and he had a similar problem whenever he re-did his brakes. He told me he went through six master cylinders until he found one that worked properly. I told them I would come back to pick up the truck later and not to do anything to it. I just don't see how that is logical - to keep replacing the same part with a brand new one until something magically works? That just seems foolish (and expensive!)

When I first start driving I do not feel as though the brakes are applied or dragging at all. There doesn't seem to be any resistance, and when the brakes first start to drag it's difficult to discern because it's an old vehicle and can be sluggish at times. The easiest way i can tell is to slow down, stop pressing the accelerator, and see how quickly it comes to a stop. Many times recently I have stopped at a stop sign or traffic light without having to apply the brakes at all. I don't think the brakes would lock up if I just sat with the truck running and not moving. It seems to start when I first apply the brake pedal and progressively gets worse, as though the pressure is increased every time I have to stop. By the time I get home (the times I have made it home) I can't even drive at idle speed and the wheel are so stuck that they won't move at all! There's been a couple of times when I wasn't able to get but within three or four feet of the curb so I put some cones in the street to avoid getting hit. I thought surely it was all four wheels locking up (and it could be - maybe the rears just aren't locking up as much because rear brakes don't use as much pressure as the first?) because the hubcaps on all four wheels get super hot. I touched them last night after it most recently locked up and all four hubcaps were hot.

The wheels spin freely when the engine is off (and cold) but once it's run and the brakes have locked up turning off the motor doesn't decrease the pressure. The only thing that releases the pressure is bleeding the brakes, but I'm not very good at doing that (especially alone) so i haven't attempted it many times by myself.

I am at a loss. I am fortunate enough to have time off until January 4th but when that day comes I need this truck to drive to work and back. I suppose my wife could take me but that's unnecessary and I have my own vehicle! There's a really good shop here in town called Saab Tire & Auto - I used to use them when I worked downtown and had something more complicated to do on my previous truck (a 1999 Silverado). If I can't figure out how to resolve this by tomorrow I'm going to take the truck there and see if they can come up with a solution. My technical skills are limited - I'm better at maintenance type stuff than major repairs, unless it involves unbolting an old part and bolting in a new one. I've replaced nearly every part I can think of that would affect this issue with all new parts, except the proportioning valve, but since that didn't fix the issue I've fished my old proportioning valve out of my scrap metal pile in case it needs to be reused. I'm thinking about doing the same with the two old master cylinders!

Let me know if you have any questions or think of any other info that would be useful. I've considered that this could be related the a power steering issue because I've read in my manual that some trucks were equipped with a booster system that is connected to the power steering, but I think that may be a stretch. I've just noticed that the power steering squeaks sometimes when I don't think it should, but it still works fine 98% of the time. Sorry for writing so much! I try to avoid being so wordy when I'm writing because most folks don't want to read so many details.

 

[Georgeb]

So if the only thing that lets the pressure off is bleeding the brakes I would try a process of elimination. With the brakes locked I would loosen the line for the front at the master cylinder to see if that lets the pressure off. Next I would loosen the line at the inlet of the prop valve then at the outlet of the prop valve and then at the inlet of each rubber hose then at each banjo bolt where the hoses attach to the calipers. Each time checking if the pressure was relieved. This may help you determine where the blockage is. Does this make sense?

 

[Georgeb]

So if the only thing that lets the pressure off is bleeding the brakes I would try a process of elimination. With the brakes locked I would loosen the line for the front at the master cylinder to see if that lets the pressure off. Next I would loosen the line at the inlet of the prop valve then at the outlet of the prop valve and then at the inlet of each rubber hose then at each banjo bolt where the hoses attach to the calipers. Each time checking if the pressure was relieved. This may help you determine where the blockage is. Does this make sense?

I left a step out. Before doing any of the above. Try unbolting the master cylinder from the brake booster when the brakes are locked.

 

[farmerchris]

does this system have a vacuum booster or is there line running from the power steering pump to the brake booster.

 

[dchwilson]

Finally fixed!!! It turns out the guy who sold me he truck (a mechanic - go figure!) had installed the wrong power brake booster. He'd installed one for a jb1 style brake system while my truck has a jb3 system. Drove it to Atlanta a few days ago (300 mile round trip) and no braking problems at all. Thank y'all for your help and suggestions!

 

[RetroC10Sport]

Finally fixed!!! It turns out the guy who sold me he truck (a mechanic - go figure!) had installed the wrong power brake booster. He'd installed one for a jb1 style brake system while my truck has a jb3 system. Drove it to Atlanta a few days ago (300 mile round trip) and no braking problems at all. Thank y'all for your help and suggestions!

All right!

 

[bucket]

I would bet that the other booster was just defective. Incorrect application or not, it shouldn't have caused the problem you were having. Glad you figured it out though!