Bleeding disc brakes issue

[James HUTTER]

installed 4 wheel disc brakes and bled them yesterday. The brake pedal became hard after bleeding but when I started the engine the brake pedal goes completely too the floor, no hard brake pedal at all. What could be the issue? Car is a 70 Road Runner.

 

[Geoff 2]

It still has air in the line.

 

[MoparLeo]

Hope this is a track car.
Rear disc are troublesome getting a high, hard pedal and contribute little to the braking on a normal street car where 80% of the braking is with the front brakes.
Only benefit of 4 wheel disc brakes is with constant braking that will heat up the brakes beyond normal. Make sure that you completely flushed out the lines and replaced with fresh from a sealed bottle brake fluid.
And you are using a new master cylinder designed for 4 wheel disc brakes.
An adjustable proportioning valve will help with braking bias.

 

[Kern Dog]

Hope this is a track car.
Rear disc are troublesome getting a high, hard pedal and contribute little to the braking on a normal street car where 80% of the braking is with the front brakes.
Only benefit of 4 wheel disc brakes is with constant braking that will heat up the brakes beyond normal. Make sure that you completely flushed out the lines and replaced with fresh from a sealed bottle brake fluid.
And you are using a new master cylinder designed for 4 wheel disc brakes.
An adjustable proportioning valve will help with braking bias.

Uhhhh, NO on many of your points.
It is not hard to get proper pedal feel with a 4 wheel disc system if the components are matched.
The front does not do 80 % of the braking. I've had rear disc pads wear out before fronts...On my 2007 Dodge truck at least. Still, a car with firm suspension and shocks doesn't nosedive on hard braking so the rear brakes actually do carry more load than 20%.
The only benefit is with constant braking? No again. Disc brakes are linear, drums are self energizing and are more prone to lockup and skidding. Rear discs are easier run just shy of the point of lockup, drums can be really hard to do the same. Discs are easier to service. Rear discs don't fade as easily either which might be the one thing I agree with.
A 4 wheel disc master cylinder is not necessary. I've never used one.
4 wheel disc systems don't need a proportioning valve, the proper systems rely on rotor and caliper sizes to dictate proportioning. You need a 2 to 1 bias to the front. The front brakes need to have double the size caliper piston surface area compared to the rear for proper proportioning.
In short, the fronts do handle more of the braking but it is nowhere near 80%.

 

[Don Frelier]

Uhhhh, NO on many of your points.
It is not hard to get proper pedal feel with a 4 wheel disc system if the components are matched.
The front does not do 80 % of the braking. I've had rear disc pads wear out before fronts...On my 2007 Dodge truck at least. Still, a car with firm suspension and shocks doesn't nosedive on hard braking so the rear brakes actually do carry more load than 20%.
The only benefit is with constant braking? No again. Disc brakes are linear, drums are self energizing and are more prone to lockup and skidding. Rear discs are easier run just shy of the point of lockup, drums can be really hard to do the same. Discs are easier to service. Rear discs don't fade as easily either which might be the one thing I agree with.
A 4 wheel disc master cylinder is not necessary. I've never used one.
4 wheel disc systems don't need a proportioning valve, the proper systems rely on rotor and caliper sizes to dictate proportioning. You need a 2 to 1 bias to the front. The front brakes need to have double the size caliper piston surface area compared to the rear for proper proportioning.
In short, the fronts do handle more of the braking but it is nowhere near 80%.

All this but you didn't help @James HUTTER with his problem.

James,

I agree that you likely have air in the system yet.
Your pedal doesn't sink with heavy pressure when it's not running?

 

[James HUTTER]

Pedal only sinks when car is running. Pedal is firm after bleeding when car is not running. Someone told me that the push rod in the booster may have too big of a gap?

 

[themechanic]

Check the brake booster.

A brake pedal that sinks only when the engine is running is typically indicative of a problem with the brake booster or its associated vacuum system. When the engine is off, the brake pedal should feel firm because there is no vacuum acting on the diaphragm in the booster Once the engine is running, vacuum is supplied to the booster, which assists in braking. If the pedal sinks significantly unde… Brake Pedal Sinks When Running

 

[WileE]

Brake bias is different with either FWD or RWD vehicles. A FWD vehicle will have a 80/20 bias, whereas a RWD vehicle will be 60/40.

 

[James HUTTER]

Pedal is firm when not running.

 

[themechanic]

Pedal is firm when not running.

As it should be.

 

[Dipstick]

Pedal is firm when not running.

Which pretty much rules out air in the system.

 

[Kern Dog]

Pedal is firm when not running.

Recently I tried to convert one of my cars from manual brakes to power. I have a few used A body single diaphragm boosters here so it cost me nothing to try. I’ve used them before and while they are not correct for a B body, they do work okay.
Three up, three down. All have firm pedal and all had short travel with the engine running but terrible action. It was as if the boosters weren’t working at all. I had no leaks, as soon as the vacuum fitting was connected to the booster, the idle was stable. I sprayed starting fluid all around the booster trying to find a leak. Nothing. I can only guess that either the amount of idle vacuum was insufficient or that somehow I had three weak or defective boosters. I’ve never taken one apart to understand their function but it might help to understand them if I did.
To answer your question, I would think that there is still air in the system. For me though, a lot of what I do is fumble around until I find a cause. It could also be the pushrod length. There are “correct” methods to get it right but I have always just adjusted it, drove the car and readjusted as needed until I was satisfied with it.