sinking brake pedal?

[going4speed]

Very odd. Sitting car off the brake pedal is rock solid. Sit at a light pressing pedal and can hear a vacuum hiss and the pedal starts to go to the floor.

What gives?

[83scamp]

Sounds to me like your booster has a leak.

[cordes]

If you hold the pressure on the pedal will it sink all the way to the floor? It sounds like you may have something going on with the booster, but it also sounds like you may have a bad MC.

[raccoon]

master cylinder internal leak. +1

[going4speed]

I thought master cylinder as well..but why is it solid when sitting with the car off? Would it not leak then too?

It does sink near to the floor. Drove the STI and tried it...sti does it too to a certain extent just not as bad.

[raccoon]

i dont know, but you should be able to stand on it without it sinking.

49345

found this on google images. I figure this is happening.

[going4speed]

would be just fine with it being the master cylinder. less excited about it being a booster. I found a aluminum master cylinder for the omni that has mini van brakes on the front.
Would that master cylinder work on the front of the lancer shelby or too much master cylinder?

[turismolover22]

With the car off, you have no power assist. A sinking pedal, or one that doesnt pump up with the car off, is usually a sign of a leaking or bad booster. You have to remember the booster adds approx 1000 lbs of pressure to the system when you push on it. So with the car on, and the pedal going to the floor, it sounds like a master cyl leak.

[83scamp]

would be just fine with it being the master cylinder. less excited about it being a booster. I found a aluminum master cylinder for the omni that has mini van brakes on the front.
Would that master cylinder work on the front of the lancer shelby or too much master cylinder?

Depends on which master cylinder is on the omni. L-bodies came standard with the smaller 7/8" bore master cylinder. Most, if not all the other body styles had the larger 24mm bore master. You definitely don't want the small bore on the bigger car. Look at the casting, IIRC, the small bore has a "7" on it, and the bigger bore has a "4" on it.

[going4speed]

Well the omni is getting SLH Brakes so that requires a bigger master cylinder. Its yet to be installed and its aluminum so that is why its high on the list of candidates. Being the omni has a ton of work left I could just put the SLH's on the lancer and the new master cylinder.


Depends on which master cylinder is on the omni. L-bodies came standard with the smaller 7/8" bore master cylinder. Most, if not all the other body styles had the larger 24mm bore master. You definitely don't want the small bore on the bigger car. Look at the casting, IIRC, the small bore has a "7" on it, and the bigger bore has a "4" on it.

- - - Updated - - -

Thanks for the detail. Amazing how well that air over hydraulics works. :nod:


With the car off, you have no power assist. A sinking pedal, or one that doesnt pump up with the car off, is usually a sign of a leaking or bad booster. You have to remember the booster adds approx 1000 lbs of pressure to the system when you push on it. So with the car on, and the pedal going to the floor, it sounds like a master cyl leak.

[going4speed]

Replaced all rubber hoses, rear cylinders yesterday. Pedal went to the floor worse after that. The rear hoses had collapsed and almost no fluid would go through them. So today I replaced the master cylinder and the pedal is solid as a rock now!!! Awesome.;)

[cordes]

Glad to hear that you fixed it before it became a real problem.

[going4speed]

I cant get over how solid that brake pedal feels now. I even started a thread asking if I was bottoming it out...its that freakin solid.
The best part of the job is using a motive products pressure bleeder. Best money ever spent there!

[going4speed]

Turns out this brake pedal still hisses in this car. Must be the booster. Ordered a vacuum pump to test its integrity.