Soft Sinking Brake Pedal
 

I have replaced the front callipers, disc's and pads, plus the rear shoes, cylinders and fluid. The pedal continues to sink to the floor under heavy braking. What next, master cylinder?

Any thoughts gladly received.

Since you already replaced everything I would recommend pulling the maste out and doing a good bench bleed only take you about an hour and if need to replace you have half the work done. But if you do replace it bleed it first you can do it yourself with a friend put it in a vise so it's level or take it to a tire service shop we do them for customers for half hour labor about $30. Good luck.

This sounds similar to my issue, but I already did put in a used master cylinder that came from another truck that had good brakes. We had chain-towed the truck home and without the engine running and no vacuum, it still stopped better than my truck.

My buddy has an 86 IDI and he is starting to develop the sinking pedal at stop signs/lights. His truck has an automatic, and he says if the light is long enough, the pedal will eventually sink to the floor and not hold the truck. They are mechanics and say it's the vacuum pump. Part is on order, we'll see what happens.

Symptoms of leaking seals in master cyl are the pedal sinks under light pressure, such as at a light. If you hit the pedal hard it usually will not sink, until you let up on it. I think you need to get a new master cyl based on your description.

I agree on needing a new master. Even if you had no vac pump at all the pedal shouldn't sink.

If you had no vac pump the brake would never sink lol. once you loose that your using the power of your leg(squats and whatnot come in handy for this) to stop. Trust me...left walmart didnt know why the brake light was on then i tried to stop lol....ended up damn near hitting someone(thank god for manual transmissions) and replaced belt next day haha.

Unfortunatly I am an amputee and my remaining leg hasn't got much power as it was badly smashed too so I rely on power assistance.

Maybe I was misunderstood or maybe I'm misunderstanding...

vacuum boost or hydroboost it doesn't matter, they only assist you in applying braking force. Neither one has an effect with the pedal slowly leaking pressure and going to the floor. You should be able to stand on the brakes holding pressure all day long, just as if you didn't have any power assist at all. Even if your vacuum booser isn't working properly, you should still be able to apply pressure all day long. If your vacuum pump isn't working, you should still be able to apply pressure all day long.

If the pedal slowly sinks when just sitting still, you're either leaking fluid externally, or it's leaking past internally in the master cylinder.

By all means keep your power assist. I'd hate to think what a chore it'd be to stop these beasts w/o any assist at all.

got the same problem, i replaced master cylinder, pads, rotors calibers, and rear shoe parts kit(all springs, and adjusters) bleed everything till i got fresh fluid out all bleeders then re-bled the system. pedal is still soft and will slowly sink to floor with constant pressure and i got week brakes. im at a loss, so i been just giving my self plenty of distance, normal stopping is ok, but emergency stopping is not happening.

Like everyone said, does like master cylinder related.


Kind of off topic question but I have to.

What vehicle are we talking about? Is it the class C camper in your avatar pic?

If so, if you can, post a large pic of it, I find that overcab bunk looking pretty interesting, never seen on with a window like that.

front rubber caliper lines and rear wheel cylinders can also be the cause for a weak brake peddle.

A bad rabs valve can cause sinking pedal just like a MC... seen it several times.

Have put up 3 pics in an album, tell me what you want to see and I'll post more.

My truck was doing the same thing a few months back and it was the master cylinder.Changed it out and the problem was solved. Thanks to everyones help on here it was fixed within a day!!!!!