Master Cylinder

[Vettefan87]

The brake pedal on my 87 has been spongy for a couple weeks. when i go to stop and apply slow pressure the brake pedal travels about half way down to the floor before starting to stop and farther down to make a complete stop, but if when i go to stop and i pump the pedal a couple of times it gets firm like it is supposed to and the car stops normal, but when i go just down the road i get the same problem could this be a master cylinder? and if there was air in the lines would it ever get firm?

Thanks

 

[vettered91]

I have the same problem in my vette and I'm almost certain it's the master cilinder, there's some brake fluid at the base over the brake booster.

For what I know; when the problem is air in the lines they feel spongy always, even if you pump it. Maybe someone here with more knowledge can guive us some light!

BTW: Maybe someone knows if there's a Master Cilinder Kit to fix it instead of buying the whole thing!

 

[taegdh]

You have to ask youselves a question or two first.

Is this the original Brake Fluid? Is the fluid more than 2 or 3 years old?

If you answered yes to either of these, first replace it. Why, because Brake Fluid, (except for the new almost pure silicone fluid), is very hygroscopic. It absorbs water. When things warm up, the water becomes steam and causes things to seem that a way, (spongy and not a very comforting sensation that you will stop as originally advertised).

Now for fluid at the base of the master cylinder. You may have to remove and check for leaks, because the "O" rings on the piston don't last for ever, and super heated water (steam) makes them more porous.

Remember, if you can't stop it, it might just ruin you whole day!

 

[vettered91]

You have to ask youselves a question or two first.

Is this the original Brake Fluid? Is the fluid more than 2 or 3 years old?

If you answered yes to either of these, first replace it. Why, because Brake Fluid, (except for the new almost pure silicone fluid), is very hygroscopic. It absorbs water. When things warm up, the water becomes steam and causes things to seem that a way, (spongy and not a very comforting sensation that you will stop as originally advertised).

Now for fluid at the base of the master cylinder. You may have to remove and check for leaks, because the "O" rings on the piston don't last for ever, and super heated water (steam) makes them more porous.

Remember, if you can't stop it, it might just ruin you whole day!

Thanks taegdh,

I answered yes to both answers and you said something that I didn't. When cold my breakes work perfect, but after ten or more minutes they start to go down so you must be right about water, steam and of course the rings.

This weekend I'll change the fluid and the "O" rings and keep you informed.

Thanks again for the nice help!

 

[The Car Whisperer]

If your brakes are spongy it's either air in the lines, a master cylinder that may be leaking internaly or you may have a leak in the sytem. If you have a leak in the system then your fluid will keep going down in the master cylinder, if it goes down too far air will enter the system through the reservoir and even if you refill it's too late. You will have to bleed the system thoroughly. If you have an internal leak the seals inside the master cylinder are leaking past and your brakes will be spongy. The leak at the rear of the cylinder is not necassarily causing the spongy problem, but if it's leaking there I would think about replacing the cylinder ASAP; that leak will only get worse. Also check to make sure the power brakes are getting vacuum. Make sure the hose is connected and that it is in good condition. Not usually a problem, but worth a quick check.

 

[Vettefan87]

I bought the car in september of last year and as far as i know the brake fluid has not been changed. I will change it and put silicone fluid back in. Does anyone know who carries this type of brake fluid and how much is it?

 

[drags1998]

Change the fluid but DONT USE SILICONE FLUID !!!!!!!!!!! It does not mix well with other fluid. Use a good quality DOT 3 or 4 fluid, it should be fine.

 

[The Car Whisperer]

Do it like drags1988 says above. Absolutely DO NOT USE SILICONE fluid!!!!!!!!!!!!!!!!!! Not compatable with this ABS system. A quart of clean fluid is enough to give it a decent flush. I run a quart through mine about every 3 years even though I only put about 1500 miles on it per year. The fluid deteriorates wether you drive it or not. Just keep flushing the system till the fluid comes out as clean as new, use 2 quarts if you have to to get it real clean. Then you should be OK.

 

[Vettefan87]

I just got done bleeding them and i got some air out of the right front. Now the brake pedal is pretty hard. If it goes back to being spongy is it a master cylinder because im not leaking any brake fluid?

 

[The Car Whisperer]

How did the air get in there? Did the fluid level get low at some point in the past few weeks, low enough to let air in through the Master Cylinder? Air should not get into the system unless something was removed and replaced or the fluid level went to low at some point in the past.

 

[vetteboy86]

Are there bleeders on the ABS unit itself?

 

[The Car Whisperer]

Are there bleeders on the ABS unit itself?

I don't think there are any bleeders on the ABS unit itself, but if you take a look at it you will see that there are lots of places to develop a leak. All of your brake lines go into this unit and come out of it. A connection could loosen, but that doesn't seem to be a common problem.

 

[tnovot]

If it goes back to being spongy is it a master cylinder because im not leaking any brake fluid?

I think you have made an accurate assumption; I would say most probably. Sounds like internal leakage past the seals/O rings, especially if they come back after pumping the brake pedal.

 

[WNH]

I bought the car in september of last year and as far as i know the brake fluid has not been changed. I will change it and put silicone fluid back in. Does anyone know who carries this type of brake fluid and how much is it?

My mechanic purged the entire system of the old brake fluid, he said the contaminants can erode the calipers, etc.
He said should be done every 5 years, but nobody knows that.

 

[malc350]

Every car I've bought has had old fluid

so I always change it. It's an easy job but no-one seems to do it :confused On my vette I use Dot 4. Bonus with a vette is you can do the fronts without taking the wheels off!

Had to do it a couple of times on my vette before the fluid was really clean as the car had been hardly used for a long time before I bought it.

My car is 90 and has no bleeder on the ABS modulator, though my 93 LT1 did have a bleed valve there - quite tricky to bleed that was, as you have to be careful not to spill any brake fluid.

 

[SPANISHVETTS]

OK, my master needs help. Should I buy a GM rebuild kit for $102 or a 3rd party rebuilt unit for $87?

 

[The Car Whisperer]

Check Ebay. I bought 2 brand new AC Delco Master cylinders for my 86 there. I paid $50 for 1 and about $60 for the other. I installed one this last winter and put the other one in storage for a spare. I got 2 new ones for less than the cost of one new one from an aftermarket supplier. Also check out The Last Detail. I think they have new AC Delco units for $119.00.