Went to take the 78 out this weekend and the power brakes were not very firm. Pedal goes most of the way to the floor. Car will stop, eventually, but the pedal is very mushey. That sound like a new master cylinder is in order?

Can I bench bleed the new master cylinder and install without having to bleed all the brakes???

 

No... you will get air in the lines at/below the line connections... so the brakes must be bled all around.

 

Mac, It's most probably not the MC, just air in the lines. Some where in your system air is slowly drawing in. Did you change out the brakes at one time?

 

Several years ago I had my calipers sleeved with ss. No leaking that I know of. The car sits 99.8% of the time. I'll drive it around the block a couple of times each month. I got in it yeaterday and the pedal went most of the way to the floor.

Is there a way to see if the MC is bleeding down before I replace it?

Thanks

 

Bleed the brakes and get the air oit, its in there garonteed. Then get the runout checked on the rotors, seems to be the most comon problem of braked sucking in air and lo liquid leaks

 

Thanks for the suggestions. Just a little more info. The last time I drove it (about 1 month ago) it was fine. I parked it for a month, when I started it (did not drrive it) I pushed the brake before putting it into gear and the pedal went to the floor. That is why i thought it must be the master cylinder since it was not moved and the pedal was soft????

 

This may be a dumb question, but, is there fluid in the MC ?
Did y'all check it recently?

 

Excellent suggestion...yes , full...Thanks

 

Reach under/around with your bare hand and feel the lowest point of each caliper, first on the inside of the rotor, and then on the outside (both caliper halves).

See if you get wet.

If you cannot reach all of them, then jack the car up and remove the wheel if necessary to gain access.

If you have standard piston o-rings (stock design), these "flat spot" over time. The weight of the piston resting on the seal causes the seal to deform & flatten out at the bottom. When the system gets pressurized again, the piston re-centers itself, and the fluid leaks out past the flattened o-ring. The fluid flows to the lowest point before dripping onto the inside of the wheel. That's what you will be feeling for.

The cure is to hang the car vertically so the piston seals don't get flattened.

 

Tom454, thanks for the suggestions. If I find brake fluid on the capiper, what is the next step?? Thanks again for your help.

 

I had the same problem when I first got my 79. I replaced the MC but it did not cure my problem. I ended up replacing the calipers.

 

There are a lot of options here, depending on your mechanical skills, available funds, and whether the calipers are already stainless sleeved.

If one is leaking, chances are they all need to be serviced, or will need to be serviced soon...
I wouldn't do just one.

The pistons have two seals.. the inner seal that seals the piston to the caliper bore, and the outer seal which is basically a dust/moisture seal. The inner seals are commonly referred to as "o-rings", although for the original square cut GM seals, this is a misnomer.

The original design calipers are not stainless sleeved.
They pitted severely over time due to water intrusion.
After honing is attempted, if the bores do not clean up,
the next step is to sleeve the bores in stainless.

Many GM calipers were poorly machined... some even mounted crooked.
I urge people to have their original calipers sleeved and returned if they are dimensionally good. I have gotten extremely poor qualilty GM sleeved calipers in return for excellent cores. I lost.... they won.
Instead of rebuilding your own calipers, You can just go to your local autoparts store and pay your money, take your chances. The quality varies widely, and you need to be selective at the counter. Hold on to your cores until you are absolutelty sure that the calipers you receive are acceptable. Once you send them in, it's to late to change your mind.

The original piston o-rings have a tendency to allow air-pumping (air intrusion) due to rotors out of spec.
The "cure" is to eliminate rotor runout, but you can band-aid it by switching the pistons out for ones with true o-rings instead of the factory style square cut seals.

You can get caliper rebuild kits and rebuild the calipers for less money.
But before you buy the kits, you need to find out what the calipers have on the inside.... stock square cut, or o-rings.

Some of the supporting vendors sell complete kits... calipers & all.
Some sell the pistons/o-ring combo's.

There are as many opinions on this as there are parts.

I'm sure we will get some "my way is best" suggestions as this thread grows.

You'll have to assess what you currently have on the car before you go forward.

First find out if there even is a caliper leak.... your MC could have gone bad instead.

 

This is only partly correct. The car must also be slowly rotated end-over-end.

 

Tom454, thanks so much for taking the time to write such an excellent response and offering the outstanding advice. The RF caliper is leaking so it looks like a rebuild is in order. I had my calipers sleeved about 10 years ago and I thought I was through with leaky calipers. I don't drive the car much at all so it looks like the seals are deteriorating.

I did rebuild them in the past so I guess I can remember how to do it. I was thinking of going with the type of seals I have in there now. I don't think they were changed to the O-Ring type back then.

Have you tried the speed bleeders? I thought I would go back with those. Any idea what size I need? Also, if I don't open the rear brakes up and only re-seal the fronts will I have to bleed the rear calipers???

Thanks again for the help!!!!

 

If the car is going to sit for long periods of time, the o-ring setup will save you some aggravation.

My C3 sat for 6 months and when I went to move it, one front caliper was leaking. There was brake fluid soaked into the pads as well.

Side note: I have stainless pistons from SSBC and they are too heavy.. causes pre-mature flat spotting of OEM style seals. If you get those SS pistons, you have to drive the car at least once a week. They should be hollow instead of solid stainless.

I took this as the perfect opportunity to test one of my theories...

If a brake pad is (DOT 5) fluid soaked, can you drive it until the braking heat evaporates the fluid from the pad ???

The answer is- yes. I drove my C3 for about 6 weeks with the brakes pulling hard to one side. Eventually, the pads dried out and braking returned to normal. The flat spotted OEM style o-ring also sealed up on its own. I was surprised. Don't recommend trying this though, because the brakes realy pulled hard... carr was hard to control. After it straightened out, I drove it to work daily from November to June.... no problems, no leaks, no pulling. The caliper & pads rose from the dead.


I purchased the "one-man" bleeders probably around 1976.
Since they were made of aluminum, and since I used them with DOT3, they corroded while sitting in my tool box. They worked fine until this happened. If you use those with DOT 3, make sure you clean them out completely after each use with brake parts cleaner.

I'm not going to comment on bleeding techinique on-line because it opens up an enormous waste-of-time debate. pm me on that if interested. I've experimented with a lot of bleeding methods and "myth busted" a lot of them. But people will still debate the results of the tests, and I don't have time for that. Too much hokus-pokus.

 

Note that when you bleed the calipers, the MC piston will travel past the "sweet spot" where it has been operating. If there is any sludge or rust buildup in the MC at that "sweet spot", it can rip up the MC seals (passing over the ridge) and cause the MC to fail... and you'll have to start all over again.

 

Come on Tom, what's the best way to bleed?

 

Cutting off a foot usually works quite well.

Saw it in a Monty Python flick.

 

reading these threads i think i will spin my plungers everytime i have a tire off and maybe keep the flattening of my OEM seals minimal