Brake Pedal gradually goes down when stopped

[Fewski]

Yesterday I noticed that my brake pedal goes down when at a traffic light or stopped in traffic. Takes about 20 seconds and if my foot is in the same position as when I originally stopped, the truck will start to slowly creep forward until I press down a little further to stop it. If I let off completely and it the brake again, the brake pedal is where it should be when pressed until 20 seconds later where it goes further down again.

Brakes are working normal as far as stopping power goes. Just seems to be an issue when I stop the brakes want to keep going down. I'm thinking brake booster? But one of the test for the brake booster was to start the truck, wait a minute or two, press the brakes down with normal pressure, turn truck off and if the brake pedal pushes back up then the booster is bad. When I shut the truck off, the brakes actually went down instead.

Doesn't appear I'm losing any brake fluid, the reservoir has the correct amount in it. Suction hose from the booster to intake doesn't have any noticeable cracks or cuts.

Any help is appreciated!

 

[turkeybird56]

Brake Booster, master cylinder leak or air in lines, but I am not a mechanic. PLS have checked professionally If U do not have the skills.

ADDED: U may be getting air in lines from a vacuum/booster line that may have a crack or bad seal that U do not see. 14 year old truck, sometimes them lines get brittle, but U neva know.

CAVEAT: BOIRD not mechanic, but may have turned a wrench 1-2 times. any doubt, professional help for sure, all IMHO...


ADDED THOUGHT: A Brake shoe sticking can sometimes give U an issue kinda similar to wat U R describing. But I think U prob got disc all the way.,. lol... Left this thought here for chits and giggles.

 

[Fewski]

Thanks for the reply. I have the skills and tools (minus the testing tools / vacuum test). Truck has only been in the shop for oil changes and tires, I've done all the other repairs (radiator, RF hub replacement, AC condenser, couple sensors, ect.). Just need someone to help diagnose it. Was hoping it could get down to one or two things and I'll replace them one at a time.
Thanks!

 

[Fast69Mopar]

Thanks for the reply. I have the skills and tools (minus the testing tools / vacuum test). Truck has only been in the shop for oil changes and tires, I've done all the other repairs (radiator, RF hub replacement, AC condenser, couple sensors, ect.). Just need someone to help diagnose it. Was hoping it could get down to one or two things and I'll replace them one at a time.
Thanks!

There will be only two main reason that the brake pedal falls while sitting stopped with your foot on the pedal and it happens because the master cylinder piston is bleeding off fluid pressure into the primary or there is a leak in the hydraulic system.

If you have recently performed any brake repairs where the hydraulic system was open and the system was not properly bled then it could cause this issue.

 

[turkeybird56]

Thanks for the reply. I have the skills and tools (minus the testing tools / vacuum test). Truck has only been in the shop for oil changes and tires, I've done all the other repairs (radiator, RF hub replacement, AC condenser, couple sensors, ect.). Just need someone to help diagnose it. Was hoping it could get down to one or two things and I'll replace them one at a time.
Thanks!

The only bad thing is: Guessnosis, always a fun thing, but just an opinion based on a lot of BS from the past. As U well know, hard to diagnose if not in front of U. Kinda like when someone calls U from 100 miles away and asks U to fix their Computer, that U cannot see screen, etc, all a symptom of Guessnosis.

 

[Fewski]

There will be only two main reason that the brake pedal falls while sitting stopped with your foot on the pedal and it happens because the master cylinder piston is bleeding off fluid pressure into the primary or there is a leak in the hydraulic system.

If you have recently performed any brake repairs where the hydraulic system was open and the system was not properly bled then it could cause this issue.

The rear brakes, both pads and rotor were replaced about 15k miles ago. That was the most recent brake job (and the previous brake job before that was easily 40-50k). No system was opened however.

 

[Travelin Ram]

Typically if you’re not losing fluid the master cylinder is worn. The piston seals are leaking, and it just goes backwards into the reservoir.

 

[Fast69Mopar]

The rear brakes, both pads and rotor were replaced about 15k miles ago. That was the most recent brake job (and the previous brake job before that was easily 40-50k). No system was opened however.

If the brake system was not opened during any repairs and you do not see any leaks from the brake hoses, line or many brake hardware then the culprit is the master cylinder as @Travelin Ram and myself said. When the piston, the piston bore or the seals in the piston wear they allow fluid pressure to bypass the primary chamber in the master cylinder. When the fluid pressure is allowed to bypass into the primary chamber the pedal will bleed down because the fluid is being released back into the reservoir.

The problem will only get worse with time so don't procrastinate with the repairs. Pick up a quality master cylinder for the repairs and bleed the brakes properly and be done. Brakes are one of the most overlooked items in the automotive world even though they are vital to the safe operation of the vehicle.

 

[BadHemi2014]

^^What they said. Master cylinder. Replace, bleed, done.

 

[CVX20]

Typically if you’re not losing fluid the master cylinder is worn. The piston seals are leaking, and it just goes backwards into the reservoir.

Yeah if he had a leak he'd have NO pedal by now.LOL.

 

[Fewski]

Just wanted to give an update (I know I love it when other people get back to what the problem was when I do research!).

So first thought by the mechanic as well was the master cylinder, unfortunately that was not the culprit. He had the good ole mechanics program that showed the possible causes (and apparently lets other mechanics 'check' which was the fix). #1 was indeed the master cylinder with about 75% of the causes. Second was the ABS hydraulic control module - which is what the problem was here. Third was a sticky caliper. But fyi, if your brake pedal goes gradually down without repumping, it's not your caliper, it'll be one of the two before that.

I hope this may help someone that runs across this thread! I'm a good mechanic and this is the first job I had a shop do on my truck (171,000 miles) but wasn't in a position to fix it this time, so no idea the difficulty for the repairs.

 

[GTyankee]

When ever, for what ever reason, if the master cylinder gets below 1/2 way empty, there is always a chance that air will get into the ABS system.
That may not have been your problem, but it is always something to check out