DaJesterMan

Hey guys,

I got a spongy brake pedal a few weeks ago and decided to work through the brake system. Heres what I've done:

Replaced a the front hard lines, replaced all four flexible hoses with stainless ones, rebuilt all four calipers, rebuilt the master cylinder, and reassembled everything after I bench bled the master cylinder.

I've tried vacuum bleeding, gravity bleeding, the 2 person method, and pressure bleeding but I am still getting the same result: a spongy brake pedal that goes to the floor when I turn the engine on.

I've also tried bench bleeding the master cylinder multiple times both in and out of the car with no improvement.

Obviously there is still air in the system or its working its way in. Ideas? I would expect that if the calipers were pumping air in, the brake pedal would start firm and soften as I pump them right after bleeding. There are no leaks when left overnight.

Thanks,
Jester

MelWff

When you rebuilt the master, you are sure there was no corrosion in the bore where the master cylinder piston seals contact?

DaJesterMan

Yep, the bore was spotless, so I order the piston kit and installed it according to an online article. I don't think its possible to install backwards.

DaJesterMan

Bueller? Anyone? Bueller?

noonie

Sounds like you have power brakes

Try carefully driving with the vacuum line to the brake booster disconnected and plugged.
If your pedal stays close to the top on braking, then its not the hydraulic system.

mrvette

I put in Hydroboost to get rid of the same symptoms.....mine did not quite go to the floor, and the car stopped ok, more or less, but what bugged me was that ~4" of pedal travel ....which was more or less the same as most GM vehicles......but with HB it's solid as hell, and travel is less than 2" to locking the brakes....

lionelhutz

Just out of curiosity, what type of fluid are you using?

DaJesterMan

If my brake booster was malfunctioning, I would expect the brake pedal not to move upon engine startup and possibly the engine to run like crap due to the vacuum leak.

DOT-3, and the car has manual steering, so HB isn't really an option yet.

When pumping the brakes with the m/c cap off, I only get the front reservoir to shoot fluid above the surface. Is this normal? Could my brake pedal/brake booster rod be out of adjustment?

Thanks for the replies so far!

micks69

I would be closely examining the rear brake lines for blockage or air leak. Did you blow out the lines with compressed air prior to rebuilding? Obviously you know the rear calipers have two bleed nipples right? Inner and outer.

lionelhutz

Do you get fluid flow when you pump up the pedal and then open a rear bleeder while holding the pedal down?

Someone posted a neat trick here you could try. Run a hose from the bleeder up the side of the car higher then the master. Open it until it fills then slowly pump the brakes. The hose might let some air out of the system.

I typically bleed by running a hose off the bleeder into a container with a bit of fluid in the container so the end of the hose is under the fluid. Then, I just open the bleeder a bit and keep slowly pumping the brakes. This lets fluid and air out but no air can get back in.

DaJesterMan

I get plenty of fluid from all six bleeders and have bled at least a gallon of DOT 3 through the system with various bleeding methods. I'll give the hose trick a shot.

What else could cause the lack of "squirt" from the rear system m/c chamber?

noonie

How far does the pedal go down with the engine off?

DaJesterMan

So I isolated the m/c by blocking off the ports with plugs and got a rock solid pedal. I then tried the front and rear systems independently and found the front system had a very spongy pedal. So definitely air in the front brake system or some other problem. Next, i'm going to disconnect each of the hard lines, plug them, and try to isolate the problem to a brake line or caliper. It did not occur to me until recently to isolate portions of the brake system to troubleshoot, so thats a good tip for people with brake issues.

Thanks for the help!

-Jester

DaJesterMan

So after many hours, I've isolated the soft pedal to somewhere upstream of the flex hoses. I used 4 line locks to isolate the calipers and when the engine is on, I still have no brake pedal. If I'm not mistaken, the brake pedal should be rock hard.

So I replaced the master cylinder and did not get any improvement. The only thing I can think of is more air trapped somewhere in the system. I bench bled the new master cylinder before installation. Is it possible a malfunctioning proportion valve could be to blame?

I'm about ready to give this car away, any ideas would be appreciated!

Jester

doorgunner

Brakes can definitely be aggravating at times.

I had good fluid flow from the front calipers....but very little fluid flow from the rear caliper bleeder screws.

I removed the small piston and springs from the proportioning (diverter valve) and reinstalled the block. Within 30 minutes I had steady fluid at all the bleed screws, and a firm pedal.

Apparently the small piston was stuck closed against the rear brake supply line.

I'll re-bleed again to remove any trapped air after making a few careful trips around the block.

my 76 ray

I want to be the first to claim it when you give it away.

dochorsepower

Just for the $.02 it worth, you could have gotten a bad replacement power booster. It happened to me. Start the car and the pedal would start to go down with each application. Crack the M/C lines and reset it and then it would repeat. I know it's a bear to replace but just a thought. There are a lot of crappy parts being made for our old dinosaurs.

mrvette

OK, first off, it's not a prop valve, it's nothing but a pressure differential switch, that if one end of the brakes fail, it tosses a switch inside, and grounds the brown wire plugged into the side....it's basically useless, in a shark the proportioning is in the caliper design...small pistons in back, large in front....so run the rear to the master directly, and put in a splitter plug in the front circuit, that will bring back a bit of pedal ....it did for ME, anyway, on my '72.....after that, you are into a hydroboost like it or not....and your better 1/2 will love you for it, as you get to put in a simple power steering box that makes a clean install....inquire about that separate thread....it's called Borgeson conversion....

the brake switch elimination fixed a bit of my soft pedal for a while....another thing you maybe try is pulling the vacuum diods/switch out of the booster, that is the whiteish plastic thing the vac hose goes to, and see if you get an inrush of vacuum, if you do the booster is 'fine' but it's NOT, it's a typical GM squishy booster that sux, and everyone knows vacuum sux.....

MrJlr

I was gonna mention the extra bleeders in the rear, but you said "all 6 bleeders".....

Don't give up! We ALL learn when you solve the problem

Chinaski

Nevermind...