Brakes, what am I missing?
#1
Brakes, what am I missing?
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
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
#2
When you rebuilt the master, you are sure there was no corrosion in the bore where the master cylinder piston seals contact?
#5
Race Director
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.
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.
#6
Team Owner
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....
#8
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!
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!
#9
Burning Brakes
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!
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!
#10
Race Director
Member Since: Apr 2007
Location: South Western Ontario
Posts: 11,061
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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.
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.
#11
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?
What else could cause the lack of "squirt" from the rear system m/c chamber?
#13
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
Thanks for the help!
-Jester
#14
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
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
#15
Nam Labrat
Member Since: Sep 2013
Location: New Orleans Loo-z-anna
Posts: 33,898
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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.
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.
#16
Melting Slicks
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
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
#17
Drifting
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.
#18
Team Owner
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
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
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.....