soft /spongy brake pedal

[billyvette]

Hey everybody, I need some advice on a soft pedal in my 78 with power brakes.
I am not loosing fluid from my calipers and the car stops ok, just with a soft pedal and IMO too much pedal travel.

What is the problem? I know my rubber lines need replacing, is the master cylinder the problem? or the booster? Car stops straight and smooth and rotors look ok visually.

Any input appreciated.

 

[Stallion]

Somebody recently had this same exact problem. I'll have to look for it. But I remember the suggestions were that it was probably the master cylinder, due to bad seals and/or leaking internally. Also, it might be the booster. But I think it's more likely the cylinder if you don't have any caliper or brake lines problems.

 

[Bob Chadwick]

I would go ahead and replace the rubber brake lines and bleed the brakes. If the system gets spongy again with no evidence of leaking lines, air is being introduced into the system. If it pumps up, a master cylinder is a good suspect. If it doesn't, you will need to go over the system looking for possible points that air could be introduced into the system. The C3 has a problem with cavitation, pumping of the pistons, at the rotors introducing air into the system. This is usually caused by rear bearings out of spec or rotors that are not within tolerances. Aftermarket rotors use better seals, VBP sells them, but it is really a band aid that doesn't fix your underlying problem.

 

[timme69680]

Before I started rebuilding my 74 I had the exact same problem, well my dad did about 11 years ago. Bob is correct in saying that more than likely air is getting into the system somewhere. In my case we replaced the booster and master cylinder, just because we didn't really know the underlying problem and the pair was cheeper than the individuals.

We found three distinct problems. The first of which were the calipers, the rubbers were completely gone, when we bought the rebuild kits we noticed that the walls were pitted. We went ahead and rebuilt them but we couldn't keep them from leaking, so we bought all new calipers. We re-bled the system and the problem was much better but not gone. The next step was to replace the brake lines. After replacing the brake lines the problem was almost gone, we thought. Finally we looked hard at the rotors and sure enough they were outside the 'good' mark. We finally got new rotors and the problem was resolved.

Without replacing your entire brake system I would look at the rotors first, then the calipers, then the presure test the brake lines, then look at the master cylinder.

 

[Bob Chadwick]

I agree with timme69680 though I would replace the rubber lines and bleed before doing anything else. If those rubber lines are original they have deteriorated and are a cheap and easy fix. They can deteriorate from the inside and will expand with pedal pressure causing the symptoms you describe. My pedal came up, though not by much, and was firmer after the repalcement.

 

[billyvette]

thanks for all the replies

When you say "lines" do you man just the flex lines or all the hard lines as well?

Thanks for all the replies, good to see stallion and bobchad again, I have been over at the classic chevy truck board as I have one of those projects going on at the same time. (2 projects at once is a bad idea for those of us who are financially challenged) I know better, but just could not help myself. Truck is a frame off project and the vette is my cool driver, until it is completely resto/modified.

 

[Bob Chadwick]

Welcome back.

I'm referring to the four rubber lines, one at each wheel, that run from the hard lines to the calipers.

I would definitely inspect the hard lines as well to make sure all is well there while I was at it.

 

[timme69680]

Sorry, I should have been a little clearer. There is really no reason to replace the actaul hard break lines unless you think they may be clogged up, which can happen. I have seen brake fluid turn into a white funky kind of substance. If your lines contain anything other than brake fluid they will make the brakes feel spungy. If you think they may be gunked up then remove the rubber lines from the hard lines, disconnect the brake lines from the master cylinder and use an air compressor to "clear" the lines of any possible gunk. It would be a very smart thing to capture the brake fluid in some sort of trap when you hit them with air. Trust me from expierence, brake fluid in your face and hair is no fun. I would definatly replace the rubber lines if they are original.

 

[billyvette]

Well, I have started on the solution.--Tony at VB and P

Just ordered from Vb and P. Tony set me up with a master cylinder, braided lines, and new o ring calipers. I hope by the time all this gets put on the 78 she stops like a new car. I just hope the booster is ok, that is the only major part I did not order.

 

[photovette]

I just came back from test driving the 'vette after replacing the front calipers, rotors, brake lines, pads, and bearings. I was bored, okay?!

Anyhow, I tried to bleed the brakes the conventional way (pump it!/hold it!) and the pedal still felt mushy after about three tries. So, I bought one of those vacuum pumps to bleed the system. It was so nice and easy to flush the whole system that I went ahead and did all four wheels. Brakes feel like brand new now. I highly recommend buying one!

 

[69Shark]

Hi photovette,

It sounds like using that device is the way to go.

I will be doing brake work on my 69 for the exact symptoms being discussed. Where did you find the vacuum pump bleeder?

Thanks,

69Shark

 

[Ripp 73]

What is the best way to know if the booster is going bad? Ever since I got the car the brakes did not kick in until I was close to the floor. Still stopping and with a slight pull to the right. I went ahead and inspected everything I could, the rubber lines look newish, the hard lines look fine, and the master cylinder is not loosing any fluid. When I am going down the road I think I hear a sqeaking sound coming from the rear and seems to go away when I press on the brakes. This makes me thing that air is being introduced in the rear. Time for new rear bearings and rotors? I still tried to bleed the brakes and they worked fine for awhile (still pulling to the right though) and then went out again. I've done that twice now. Also, when ever the car is cold and I press on the brakes I hear a pssssss sound. That makes me think its the booster. I don't have much funding to replace everything right now... what to do?

 

[billyvette]

Update, All the new stuff mentioned in an above post is now installed along with the rotors trued, front wheel bearings repacked, and new front seals. Total installation charges was $433. Before, all you guys start, I am a brake novice and do not have anyone to so much as to pump the brake pedal, so I took it to my repair shop that I trust to do the work. VBand P sent 2 left rear calipers and I had to get a right rear caliper sent. Tony thought I was going to be very angry, and I was not. He is a professional in every sense of the word and I will definitely do business with them again. My only complaint, was a lady told me it could take 30 days to determine my core refund, I think that is too long; due to the credit card cycling before they may credit me back. This is a very minor point although. Car drives better with repacked wheel bearings and stops very well. Use VB and P with confidence, I will again.

 

[Bob Chadwick]

What is the best way to know if the booster is going bad? Ever since I got the car the brakes did not kick in until I was close to the floor. Still stopping and with a slight pull to the right. I went ahead and inspected everything I could, the rubber lines look newish, the hard lines look fine, and the master cylinder is not loosing any fluid. When I am going down the road I think I hear a sqeaking sound coming from the rear and seems to go away when I press on the brakes. This makes me thing that air is being introduced in the rear. Time for new rear bearings and rotors? I still tried to bleed the brakes and they worked fine for awhile (still pulling to the right though) and then went out again. I've done that twice now. Also, when ever the car is cold and I press on the brakes I hear a pssssss sound. That makes me think its the booster. I don't have much funding to replace everything right now... what to do?

Mine pulled as well. Brake job and new tires made that go away. Could be a sticking caliper or a problem tire.

The pedal to the floor seems about normal though mine came up some when I changed out the rubber lines.

The squeaking could be a lot of things. E brake, sticking caliper and bearings for starters. You will need to take a close look at this.

If the booster is going bad, you will need a lot of leg power to stop the car.

Sounds like you are getting air introduced into the system from some point.