rear brakes are not working but the front brakes are

[marandall]

I have a 1979 Chevy k-20 with disc brakes in the front and drums in the rear. the problem I am having is that I am not getting any fluid to the rear brakes if I pump long enough a little bit of fluid will be present along with some air bubbles from the pressure fitting but it's never a lot of fluid coming from the fitting. I cracked the lines going to the proportioning valve and fluid comes out from all ports when pedal is depressed. so what I would like to know is there a certain braking bleeding procedure or does the proportioning valve need an adjustment or if anyone knows what I am doing wrong. Advice is surely needed   

[zieg85]

Are you sure you are getting fluid out of the proportioning valve line that feeds the rear brakes?  If so the rubber line may be collapsed that feeds the lines on the differential.  If not the valve may be shifted to the front in which you have to bleed the front brakes to shift the valve back to the center.  It is also a possibility that the master cylinder could be bad and leaking internally just on the rear section.  Check the rear bleeders to make sure they are clear.

[marandall]

I have already bleed the front brakes several times and they are working great I jacked the front wheel up and with the wheel  spinning by hand you can hit the pedal and the tire stops spinning. I cracked the line that comes from the rear side of the master cylinder and goes into the valve and fluid was present at both ends when pedal was pressed. then after the lines were tighten down i went to the other side of the valve where the fluid goes to the rear lines and cracked it there and fluid was present when pedal was pressed and the pedal starts to get soft. but once i tighten the line back up and hit the pedal twice it is firm and still no fluid present at the rear wheels 

[zieg85]

When did this problem start happening?

[marandall]

as soon as i replaced part of the rear brake line. when i was cutting the cab bolts out my grinder slipped from my grip and hit the brake line and cut it. before  i could get the line off all of the fluid was drained out of it

[zieg85]

OK, you have a massive amount of air in that line and it will take time to clear.  If it were me I would open both bleeders in back and gravity bleed it.  I've also used an old master cylinder lid and installed an air fitting in it with a low pressure air regulator with 1-2 psi on it to help push the air out.  Make sure you keep the fluid level up.  Did the master run dry on the small reservoir?  You may need to bench bleed it if it did.

[marandall]

what do you mean by gravity bleed?. both reservoirs were dry when i opened the lid.

[zieg85]

Take the lid off the master cylinder, fill it up.  Open the rear bleeders and gravity should take over.  It will take some time but if the bleeders are not stopped up and the rubber hose isn't restricted the fluid chases the air out.

[marandall]

ok i will try that. another thing that i noticed. Is when i have the lid of the mc the pedal is able to move pretty easily but as soon as i install the lid and clamp it down the pedal becomes really stiff and hard like the brakes are brand new but still no fluid is present at the rear wheels. would that be because of the air in the system.

[zieg85]

I've not personally experienced that what you describe.  I wouldn't think there should be a difference whether the lid is on or off once the vacuum is gone from the power booster.

[marandall]

yea it was puzzling me and my dad when we first noticed the difference. thank you for you suggestion and help the sun is starting to go down at my house and since the brakes are not working prob i have the truck outside instead of my garage. so i will have to try you gravity bleed method tomo. i will keep you posted about my progress

[zieg85]

No problem, hopefully you can get this sorted out.

[marandall]

yea hopefully i have already swapped the cab, bed and the engine in the truck and i am on the finally leg of getting the brakes and electrical working properly

[Fairlane514]

Also remove the bleeders completely from the wheel cylinders and make sure they are not clogged with dirt etc...