Inconsistent spongey brakes: 72 280 SE 4.5

[tviano]

Ok. Here's the deal. I'm at a stoplight. Engine is idlling nice and low. Car is running great. Then, with my brake pedal pushed, the car starts to creep forward. I push more on the brakes, and sometimes it feels as if I'm nearly near the floor "boards."

My mechanic said it's just the front brake pads that need to be changed and this spongey-ness is because the pads were worn and im pressing on the springs in the brakes.

So, I had him change the brake pads, and it started doing it again today.

Could this be air in the lines?

The master cylinder--according to my mechanic-- is fine. When the engine is off, there is tension on the peddle. Not spongey at all. I'm not losing any fluid either.

HELP! This one is concerning me as I think brakes are an important part of any car!

[Mark DiSilvestro]

I've experienced a problem where the car comes to a stop just fine but, while waiting at a light, the brake pedal sinks towards the floor. Sometimes it will pump up and hold. Worst case, it won't. With no external brake fluid leaks, it's probably a faulty brake master cylinder. While applied, the brake fluid bypasses the master cylinder piston seal, back to the reservoir. With the engine off and no power assist, it's more difficult to repeat this condition.
Hope this helps.

Happy Motoring, Mark

[tviano]

So tell me, did you replace the master cylinder? I'm assuming this is a big job.

If you replaced it, did you replace it yourself or have somebody else do it?

And, do you think my mechanic missed the mark?

THANKS!

[gmask]

Originally posted by tviano
So tell me, did you replace the master cylinder? I'm assuming this is a big job.

If you replaced it, did you replace it yourself or have somebody else do it?

And, do you think my mechanic missed the mark?

THANKS!

It's not a big job. rebuilding the cylinder is more complicated but simply replacing it should only be a few hours work for DIY.

[psfred]

It is also possible that the booster is leaking down while you have the pedal depressed, giving you a VERY hard pedal. If it doesn't sink, just gets very hard, booster. If it sinks slowly, master cylinder.

I have to use tremendous pressure to stop mine (in the 50 yards or so I've driven it) mainly due to rust on the rotors, I think. I have a rebuild kit for the master cylinder, as mine was leaking out the weep hole at the rear bottom side (and there is no paint on the bottom half of the booster). Will do that and bleed, probably pull the wheels and check pads, too, once I get the engine to hold water.

The vac hose can be bad, too -- they get hard and brittle with age. Ditto for the check valve.

Vac leak should give you a racing engine, too, as this one will add fuel rather than go lean, so maybe you have the engine pulling you , too!

Peter

[Mark DiSilvestro]

Thomas,
"Spongey pedal" could be air in the system but you said nothing about brake problems other than while stopped at a light. The pedal going to the floor while stopped indicates an internal leak in the master cylinder since you said there was no loss of brake fluid.
If the booster or its' vacuum hose fails, you get a hard pedal that requires a lot of pressure to stop, not a spongey pedal.
A vacuum leak at the booster will cause a lean mixture to one or more engine cylinders, like any other major vacuum leak. (I just replaced the failed booster on my '72 250. It caused a hard pedal, much higher pressure to stop at all times and a miss from # 6 cylinder)
I would probably bleed the brakes first to see if there's an improvement. If not, it should take less than an hour to replace the master cylinder.
As for the mechanic, he should have checked the car out AFTER he did the work.

Happy Motoring, Mark