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Brake pressure build up

Here’s one that no one can figure out. I hit an unmarked median in the dark at 40mph in my 1991 Camry 6cyl. My transmission was destroyed. I replaced it. Now my brake presure keeps building up after frequent brake use in a short period of time until the car just stops moving. This is less of a problem if I use the brakes infrequently like on an interstate highway. I bought a used car to get the transmission and then used the brake parts on it to swap out with my original car: front brakes, calipers, pads, rotors. flex-lines to the calipers, and master cylinder - brake presure still increased with constant use (like driving in heavy traffic). If I feel the brake presure building and the car starting to drag I pull over and park for 2-3 minutes and the brakes release. Mechanic friends, dealer mechanics, and brake shops can’t figure out why the brake pressure builds up after frequent stopping until the car comes to a stop and smoke comes from the front brakes. I backed off the plunger in the brake booster a couple of times with little change. Today I wiggled the plunger in the booster when I pushed it back into the booster after I adjusted it and felt the plunger settle back a little farther, like it was settling into a washer or a hole or something. Then the brake pedal went to the floor. I adjusted the plunger back towards its original position and the pedal still went to the floor. Can it be that when I hit the median the plunger jumped forward onto the edge of a washer or hole it normally sets into and that kept the plunger pushed forward, thus not allowing the fluid to return from the calipers? And then when I subsequently adjusted the plunger I never really let it settle into the inner parts of the booster? And now do I need to adjust the plunger back further toward it’s original position, making sure the plunger settles into it’s little hole(?) until the brakes work like they used to? Figure this out - no one else can!

Have you ever heard of paragraphs? Your post is VERY difficult to read, since it just goes on and on with no breaks.

Having said that; You’ve spent all this time and effort on a '91 Camry?



You must really love this car.

I have no idea what’s wrong with the brakes on your car, but I admire your dedication, and I wish you the best of luck.

How about replacing the brake vacuum booster?

I’ll bet the flex lines are bad. They can break down from the inside out, and when the inner liner breaks down, it can act like a check valve, preventing the pressure from releasing the brakes. I would have never used junkyard flex lines, but bought new replacements. You may have to do that now.

Suspect internally split brake lines.

It has to be the booster because I have swapped everything else with no change. I will swap the booster this week. I haven’t done that yet because I was afraid the mounting bolts under the dash might be hard to get at. Thanks for the reply.

I swapped the flex lines and no change.

I assume you are talking about the flex lines connecting to the calipers. A mechanic friend suggested that so I swapped the lines with the ones on the car I bought for the trannsmission. No change. It’s hard for me to believe that the 2 original flex lines and the 2 from the used car are all split causing both front brakes to slowly lock up simultaneously. But . . all suggestions are appreciated.

Check for free play between the master cylinder piston and the booster output rod. You can simply loosen the nuts holding the M/C to the booster about 2 turns and try to repeat the lock-up condition.

I’m inclined to suspect something amiss in the master cylinder or lines. When you release the pedal, the pressure should dissipate back through the system, past the MC pistons, and back to the reservoire. It sounds like the pistons or damage to a line are/is not allowing the pressure to fully dissipate back into the MC. While the plunger in the booster could,as you stated, be holding the linkage in as if you were still pressing on the lines, damage to a line seems much more likely.

Or perhaps even mechanical damage to a cylinder itself. Has anyone pulled the cylinders and tried pushing the pistoms back with the bleeders open?

You mean calipers?

I believe the same thing that something is restricting the fluid return from the calipers. I pushed all the pistons on all 4 of the calipers I used on the car and they all were easy to push. Since swapping the master cylinders produced no change I kindof discounted that but I may have to buy a new one if nothing else works. Everyone I have talked to discounts a problem with the booster but it is the only brake part I have not changed out. Yesterday when I readjusted the plunger on the booster for the 3rd or 4th time I was suprised that the brake pedal went to the floor and would not pump up and that hasn’t happened before. When I adjusted the plunger back to where it was before the last adjustment the pedal still went to the floor. The only thing I noticed different was that when I pushed the plunger back into it’s hole after adjustment I had to wiggle the plunger a little to get it to settle back into what felt like a loose washer in the booster so that it would go all the way back into it’s hole. That makes me think something in the booster broke loose when I hit the median and slammed on the brakes. I’ve been a pretty good shade tree mechanic for 40+ years and I don’t ever remember having to replace a booster but I swapped out everything else. Now that the temperature in Atlanta has dropped to 15 degrees I’m loosing my ambition to work on the this car for a couple of days. Thnks for the reply. Do you know of a exorcist that works on cars?

You sound like an excellent shade tree mechanic.

15 degrees? I wonder if you have ice in the lines. Have you been doing the work outside?

Just a wild thought…condensation from replacing the parts outside?

I’ve been working on this for a couple of weeks, sometimes in warmer weather or in my garage. The lines haven’t frozen but the carpeting in the car did when I shampooed it and left it out overnight.

I readjusted the plunger on the booster for the 5th time and now the brake pedal goes to the floor. So i bought a remanufactured master cylinder and the bedal goes to the floor . . but I did notice the plunger sat back into its hole at least a 1/4 inch farther than before. The only thing I haven’t replaced is the booster to I am gathering up a new batch of 4-letter words and will change out the booster today. I don’t remember ever fooling with a brake booster in the last 40 years. I guess it’s my time.