Brakes!! Losing pressure. Help!!!!

[kpres419]

I have a bit of a Frankenstein truck. It's titled as a 73 Chevy C10 with a 86 body. My ? Is my brakes were working fine then all of a sudden they started losing pressure. They will pump up fine then while your foot is on the pedal it will slowly fall to the floor like someone cracked the bleeder valve. Then I will pull the pedal back up and pump them up and they work again. The fluid is fine so it's not losing fluid anywhere I've checked for leaks. I was trying to search for similar problems on here but couldn't find any. I'm thinking master cylinder but not sure. Could this be the brake booster?  Any help is greatly appreciated and thanks in advance. 


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[Chisholmcody]

I'm going to say your master cylinder is going bad but wait for others to chime in


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[bd]

If you've verified there are no fluid leaks from the rear wheel cylinders, front calipers, or along the entire length of the brakes lines and hoses, side to side and front to rear, AND the fluid level is not dropping in the master cylinder, then the master cylinder is bypassing internally. 

If there are no visible brake fluid leaks, but you are needing to add fluid to the master cylinder, the M/C is leaking into the booster.  In either scenario you will need to replace the master cylinder and flush the system with fresh brake fluid.

[rich weyand]

I had the exact same thing.  It was the master cylinder.  There are three rubber seals along the piston.  Rubber age hardens, and then is not supple enough to provide a good seal.  If it is the rear seal on circuit 2 inside the master cylinder that leaks, you will be losing fluid into the booster.  If it is either of the other two seals, you will be losing fluid back into the reservoir.  Either way, the pedal will fade to the floor as you sit with your foot on the brakes.

Nothing you did to make it happen, or could do to prevent it.  Rubber gets old, and it gets hard.

Replace the master cylinder first.  Easy enough.

[1986GMCSierra]

I had the exact same thing.  It was the master cylinder.  There are three rubber seals along the piston.  Rubber age hardens, and then is not supple enough to provide a good seal.  If it is the rear seal on circuit 2 inside the master cylinder that leaks, you will be losing fluid into the booster.  If it is either of the other two seals, you will be losing fluid back into the reservoir.  Either way, the pedal will fade to the floor as you sit with your foot on the brakes.

Nothing you did to make it happen, or could do to prevent it.  Rubber gets old, and it gets hard.

Replace the master cylinder first.  Easy enough.

i have a question about my brakes... do i need to bleed them if i have a leak in order to put more fluid in it...... also my truck was sitting for 10 days with the parking brake on and when i released it after that time the light stayed on and it felt like the rear brakes were sticking.. but not when it was in reverse... thanks

I'm going to say your master cylinder is going bad but wait for others to chime in


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If you've verified there are no fluid leaks from the rear wheel cylinders, front calipers, or along the entire length of the brakes lines and hoses, side to side and front to rear, AND the fluid level is not dropping in the master cylinder, then the master cylinder is bypassing internally. 

If there are no visible brake fluid leaks, but you are needing to add fluid to the master cylinder, the M/C is leaking into the booster.  In either scenario you will need to replace the master cylinder and flush the system with fresh brake fluid.

[Irish_Alley]

well air has replaced the fluid so yes you need to remove the air. the parking brake light switch is on the pedal itself, so if the lights on your pedal isnt all the way up. sometimes they still stick and if you drive backwards they can unstick themselves. ive had to shake the cables by hand to get some to release

[Dr_Snooz]

Assuming you didn't let the master cylinder run dry, and don't have a spongy pedal feel, you're probably okay to let the bleeding go for a small leak. A big leak should be corrected, of course, followed by a proper bleeding.

If moisture gets into the brakes, you can get corrosion that glues the shoes to the drums. You can also get patches of rust on the exposed portions of the drum that make the brakes pulsate and feel grabby for the first few stops. You can also get corrosion on the brake hardware, which requires a tear down to wire brush and re-lube. Corrosion or kinks in the brake cables can also cause problems.

Pedal slowly falling to the floor when stopped at traffic lights is classic MC failure. Brake fluid leaking into the booster often goes out the tailpipe as white smoke. Booster failure usually produces a rock hard pedal feel and brakes that require much more force than normal to stop the car.

[1986GMCSierra]

Assuming you didn't let the master cylinder run dry, and don't have a spongy pedal feel, you're probably okay to let the bleeding go for a small leak. A big leak should be corrected, of course, followed by a proper bleeding.

If moisture gets into the brakes, you can get corrosion that glues the shoes to the drums. You can also get patches of rust on the exposed portions of the drum that make the brakes pulsate and feel grabby for the first few stops. You can also get corrosion on the brake hardware, which requires a tear down to wire brush and re-lube. Corrosion or kinks in the brake cables can also cause problems.

Pedal slowly falling to the floor when stopped at traffic lights is classic MC failure. Brake fluid leaking into the booster often goes out the tailpipe as white smoke. Booster failure usually produces a rock hard pedal feel and brakes that require much more force than normal to stop the car.

thank you for the help i will bleed tne brakes now before it has the chance to go really bad....

well air has replaced the fluid so yes you need to remove the air. the parking brake light switch is on the pedal itself, so if the lights on your pedal isnt all the way up. sometimes they still stick and if you drive backwards they can unstick themselves. ive had to shake the cables by hand to get some to release

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