I have a weird brake issue. When I press on the brakes, they are weak, and only two brakes are getting fluid. Also only two brakes bleed at all.

I took off my Master Cylinder and Brake Booster. I undid all of the brake bleeder screws and i took my air compressor and blew out all of the brake fluid in the lines. Every tire had fluid come out fine when I did this.

I put on a new booster and a new bench bled master cylinder. I hooked everything up and re-bled the brakes.

The weird thing is only the drivers side rear and the passanger side front bleed ok for one cycle but then nothing comes out at all. The other two tires bleed fine all day.

Does it sound like it could be the proportioning valve.

Again here is the basic issue. Weak Brakes.

Drivers Side Front - Bleeds.
Drivers Side Rear - Does not bleed.

Passenger's Side Front - Does not bleed.
Passenger's Side Rear - Bleeds.

 

It sounds like you have a leak at some point in the system, and you're right, at the master cylinder or at the prop valve is the most likely area to be flawed. Most people would do something like "replace the prop valve", correctly tighten the hardlines, and assume that replacing the prop valve was what solved the issue. I doubt the prop valve is the problem, but the lines are definitely going to be an issue.

It happens like this: Hardline seeps brake fluid when you press the brakes. When you let off the brakes, the loose fitting and super-high vacuum inside the line causes bubbles to pull back into the line.

This can also be a major problem bleeding brakes at the wheel. What I do to avoid this is, pull on the bleeder valve outwards to keep pressure on the threads and prevent air from seeping back into the system without passing through the bleeder tube. I also tend to not loosen the bleeder screws very much so the air that wants to bypass the bleeder tube has a more complicated journey to travel.

 

I already bought the valve and MC to replace. After I put them on, is there a way to check pressure in a brake line?

 

I had a faulty master cylender that only put out fluid on front outlet which feeds the right front and left rear. The outlet on the side had no fluid coming out. I replaced the master cylender but am having trouble bleeding rear brakes. There is no fluid coming out of the proportioning valve for the lines going to the rear. The front brakes have basically straight through channels but the rear have some kind of safety valve that shuts off if there is a leak. Does anyone know how to overcome that vavle? I can bleed front brakes ok but not either rear brake.