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?