The other day i switched rears on my car. I used to have a drum open rear and now i have a posi rear with discs. I changed the propertional valve to the disc brake one and i bled them. Now sometimes all four of my brakes work and other times only the front brakes are working. Im thinking that maybe i did not bleed the brakes properly, or could there be something else wrong? thanks for any help.

 

Most RWD cars that have front disc and rear drum have metering valves in the front brake lines. This valve controls the timing of pressure application to the front discs. This helps acheive balanced braking between the front and rear. It needs to do this cuz disc brakes are 'fast-acting' whereas drums have to overcome spring tension and linkage clearance.

All the proportioning valve does is controls the pressure. If you applied the same pressure tj front discs as you do rear drum, the rears would lock up. This happens cuz front disc require a lot more pressure to apply them than drums do. So all the p-valve does is regulate pressure between the front and rear brakes so your rear brakes dont lock up.

Is it throwing your brake warning light on? Does the pedal pulsate? When is it doing this problem? Hard or soft braking?

 

Thanks for the help joshp14. My brake light does not come on and the pedal does not pulsate. As for as when it happens, it just happens like here and there, its weird b/c sometimes when i push on the brake i can feel all four wheels braking and then other times only the front brakes work. The pedal is kinda soft feeling when pressed down, but thats b/c of the master cylendar right?? i read that it is not nessesary to change the master cylendar when you change the rear from drums to discs, but if thats the problem then thats what i will have to change. Again thanks for any help you can give to me.