Disc Brake Troubleshooting

The problem:
Just installed new 4 wheel disc brakes all around, have run about a gallon of brake fluid while bleeding through the system, I did bench bleed the Master cylinder(3 times on the bronco and the last time with a bench vice). The pedal is still "squishy" will bottom out on first stroke and gets hard with about the third push. After about 10 seconds the system loses pressure and it takes another 3 pumps to get a good pedal feel.

I'm at a loss I have no idea why this isn't working and am literally about to push this thing off the nearest cliff! Any advice to help prevent me from getting rid of my project is welcome.

Here are the stats: F150 disc brakes up front w/ thunderbird calipers, Chevy disc brakes out back from TSMMFG.COM, new(rebuilt) 4 wheel Manual disc brake Lincoln Mark V master cylinder, with stock 1974 Bronco Manual push rod, new rubber hose all around, stock H block as the splitter.

Thanks in Advance,

Chris

Check for any possible leaks, could be sucking air from that point? That line about bottoming out on the first pump imo sounds like a bad master cylinder. Keep bleeding from the farthert point. Good luck... Scott
P.S. Welcome to the site..

Sounds like you've got a leak. Check the lid again.

i was in the same boat last month with the chevy swap finaly took it to a local brake masters and had it power bled $89.00 but worked great after it was done

Hey Chris,

1. Is the fluid level staying the same?
If yes, you might be sucking in air into the system.

2. Is your master new or rebuilt? I've had bad masters that were rebuilt in the past.

3. You might post asking everyone how to properly bench bleed a master, I'd change the master but. . .

4. Are your calipers new or rebuilt? Old calipers can get like a bubble on the rubber seal. You won't loose fluid but the petal will not stay firm as the bubble inflates and deflates. I had that on my 67 Mustang after it was parked for a few years.

5. Hydro or vaccumme booster? I did try vaccume but it didn't do a good enough job, the hydro booster is amazing.

6. Have someone pump the pedal and go look at each hose connection and at the calipers. Try to see if anything is expanding or bubbleing out. Also listen for air getting sucked into they system, doubtful it's sucking but the problem does suck.

7. Test drive and stop hard on it alot before you hit the streets.

8. If all else fails, go ask a mechanic but don't come across like a know-it-all, maybe he'd think of something we are missing.

Good luck and keep up the efforts! Summer is almost here and it's soo worth it!

Chris in Los Angeles
taipeichris13@yahoo.com

Pump up the brake system and then hold firm pressure on the pedal for about ten seconds. Check for any leaks at the wheels and along the frame. I you have a vaccum booster check for brake fluid coming down the front of the booster. It really sounds like you have a ad mc, if the seals around the piston in the mc are bad or if it is pitted out you could be loing pressure internally to the mc.

I would first verify that the master cylinder is in proper working order by plugging both outlet ports and stepping on the pedal firmly. The pedal should be solid once it hydraulically locks and should not sink over time. If it is "squishy" then it still has air in the MC. If it drops over time, then you're leaking fluid internally past the seals and back into the reservoirs or it's leaking externally and into your firewall.

Assuming the master cylinder checks out, the next step would be to go through the system one fitting at a time to make sure that everything is dry and you have no leaks between the MC and the calipers. Leave either the front or the rear MC outlet port plugged and hook up the other hard line to the MC so that I could determine if the problem was front or rear. I'm guessing rear since you said you are running the Chevy(?) calipers...it would help to know if you're running the Cadillac/Pontiac parking brake calipers, as they tend to include several other initial installation issues due to the parking brake nuances.

Once at the calipers, verify that the bleed screws are up so that they can bleed properly. Different caliper castings for different applications located the bleed screws differently, so look at the porting of the bleed screw, not just the screw itself. It's where the port intersects the piston cavity that should be located at the highest point, not necessarily the screw itself.

You've got a fairly simple system, so with some methodical troubleshooting, you should be able to find the problem and address it or them one at a time without too much difficulty I would think.

Tobin

Thanks everyone for the suggestions. I'm going to start with my master cylinder since it is rebuilt.

Apogee - They are the chevy monte carlo calipers and they don't have the parking brakes. Also the bleed screws are pointed up.

Thanks again everyone!

Did you flare your lines? And if you did, did you double flare them? The same thing happend to me and i tryed everything at the end i found out it was the lines that i had flared, they must be double flared!!!

Swap the calipers side to side so the bleeder nipples are on top. You can never get the air out if the nipple isn't on top.