Greetings all!

Working through some brake issues on my 81. Bled the system, pedal is hard and confirmed that brakes are indeed working (tires won’t spin). As soon as I start the car up, pedal goes soft and has absolutely no resistance. Anybody have an idea as to what I could be missing?

My only other idea is to try and keep bleeding the system, but with the pedal already hard it’s difficult to see how that would help with getting a hard pedal once the car is on.

many thanks!

 

You replaced the master cylinder?

 

Yes, new master and booster (measured correctly and an absolute pain to install). Calipers replaced as well as upgraded to SS flex hoses to each wheel.

 

Are you using Motive to bleed it?

 

No, I did gravity and then the ole 2 man method. Got a reverse bleeder I can try too. I took some advice from a previous forum and plugged the vacuum line to the booster, and brakes stayed hard after starting. As soon as I reconnected it to the booster, they went soft again.

 

Gravity bleeding works great for flushing out a system that is in good shape. If the proportioning valve or master cylinder is not functioning correctly, gravity bleeding is ineffective.

I'd avoid the 2 man method. General consensus around here is "don't touch the brake pedal".

First step to bleeding. Remove the master cylinder from the car. Clamp it to your bench. Buy a master cylinder bleeder kit. I use old brake lines. Basically, two fittings with hoses that screw into the side of the MC. Other end of hose is submerged into the MC reservoir. Pump the master cylinder until no air bubbles are present. Only use 1" strokes, pushing too far can damage the MC.

Now you can start the bleeding process. 2 bleeders up front, 4 in the rear. Rear has inner/outer on each caliper. Believe you start with the inner, then outter. Typically start with the farthest caliper from your MC.... truthfully, doesn't matter.

Reverse bleeders can introduce air into the system at the fitting. I prefer a dry Motive Bleeder hooked up to an air compressor.

Also jack your car up until the master cylinder is level. If your brake light is on... stab the pedal a few times to bring the proportioning valve back around.

Once its properly bleed, it hopefully resolves the brake pedal issue.

 

Just installed a Wilwood master cylinder on my 80. Bench bled the master cylinder prior to install. Used a Phoenix reverse bleeder starting at passenger rear caliper. Entire project took about 2 hours.

 

Get, or make, a pressure bleeder. I don't even try without one. And now --- I even do any any other vehicle with my pressure bleeder.

 

$45 for the adapter. Screw some fittings on it and hook it up to your air compressor at 10 - 15 psi. You'll quickly get tired and annoyed pumping the pressure tank by hand. Most of us prefer the dry method, so we don't have to clean out the pressure tank.