Need help w/ master cylinder

[68jimmy]

Well I'm almost done with my disc brake conversion and have run into one more snag... Mr Murphy has been hard at work in my garage

The power booster master cylinder and prop valve were taken off of a 72 parts truck I have. The bleeding of the front brake went ok with no issues. Now when I started to bleed the rears it just seemed that little to no fluid was moving. So I went to the master cylinder and tried to bleed from the fitting just above the hard line that goes to the prop valve. After I had my wife pump up the brakes and I crack open the bleeder what come out is frothy and not much fluid. And the pedal does not go down... When I do the same with the bleeder for the front brakes it I get plenty of fluid with no bubbles. I then removed the hard line from the mc to the prop valve and it's still the same. Do I need a new mc? Or should I check something else?

In the attached pictures the hard lines for the brakes are as follows: The left on the mc feed the front brakes and the right line to the rears...

Thanks in advance!

[JimKshortstep4x4]

You may need another master cylinder but try loosening up the master cylinder to booster mounting screws. Loosen them up until you are certain that the booster rod has play in it, (if the rod has the piston in the master cylinder depressed than it will not let fluid into the rear part. Also, make sure that the port in the rear of the master cylinder to the piston is not plugged.

Jim

[Jimbotronics]

Did you bench bleed the m/c? Not an expert on brakes by any means but have heard not bench bleeding can cause problems.

[Green Machine]

Had the same problem with our 71 3/4 ton. Put on a new master cylinder and no fluid to the rear.
The problem was the proportioning valve was stuck. We removed it and took the nut off of the end and disassembled what we could. Cleaned out all the gunk and washed it out with WATER. Don't use any solvent or cleaner here. Blew it all out with air and reassembled. It works just fine now. It likely should be replaced, but it seems to work ok now.

[c-1072]

I would replace the master cylinder . I have seen them stick internally specially if the sit for a long time . i bought a new one 15.00 at advance auto .

[68jimmy]

That's what I'm thinking... the truck sat for probably 3 years before I got it and I've had it since February sitting in my yard... So it's probably due for a new mc... Thanks for all the replies!