[weim55]

Hmmmmm, you took it to a shop that does brake work professionally and they didn’t figure it out ?! Wow, do professional shops even say the word diagnose anymore ?!

Anyway, should be a pretty simple diagnosis. Remember, start at the first point of fluid ( The master cylinder) and component by component work your way to the rear of the truck. Remember, EVERYTHING is suspect and should be checked including all the new parts. ALSO, this is a 50-year-old truck so it’s quite likely you could be dealing with a compound problem. If I had your truck in my shop I would start by quickly trying to isolate the problem from the front or rear, or both as being part of the issue.

Disconnect the line from the proportioning valve that goes to the rear brakes, next open the bleeder valve on the wheel cylinder on the right rear, if you can connect a hose into a jar to collect fluid and reduce spray with the following test : using compressed air blow into the line you disconnected at the proportioning valve and see if you have flow to that right rear wheel cylinder. If you have flow switch sides and check the left rear. If both are good the next check will be the master cylinder to the proportioning valve. If you are not getting flow to the wheel cylinders here is your next flow chart things to test in the order of what is usually most problematic first:

Remove the bleeder screws from both wheel cylinders and see if they are clogged. Very common for these to be full of dirt and crud from 50 years of road use. Next disconnect the hard line that goes from the proportioning valve to the rubber hose at the rear of the truck. Disconnect the fitting right where it goes into the rubber hose. If there is a clog in the line this is the number one place were usually occurs. Now try blowing compressed air from the disconnected line at the proportioning valve to the rear of the truck and see if you have flow. If still no flow I can almost guarantee you the problem is going to be in that rubber line. Usually the left and right hard lines on the rear axle will be OK., You have it all apart at this point good idea to blow out each component. Now that you have checked everything from the proportioning valve to the rear of the truck next to check everything from the master cylinder to the proportioning valve.

All new parts are highly suspect so check each one starting with the master cylinder. If flow through this master cylinder itself is good if you disconnect the hard line to the rear from the master cylinder fluid should easily leak out of that port via gravity. If no flow, something is clogging the Feed hole in the bottom of the reservoir or the second most likely problem, the piston is not returning to its full neutral position because I’ve been out of adjustment or incorrect pedal push rod ( or incorrect master cylinder all together OR A power break booster that is misadjusted or incompatible altogether. It’s a master cylinder checks OK next check for flow through that short line that goes from the master cylinder to the proportioning valve. If that checks OK, next on to the proportioning valve itself. You made mention you are not getting flow here. I have had problems with aftermarket proportioning valve’s twice. The valve gets tripped and is unable to return to the neutral position. Even when I removed The proportioning valve and made an attempt to RECenter the piston on the workbench it was impossible. I cut open the second problematic brand new proportioning valve and found the piston completely stuck inside the body. The piston and body were not machined, sized properly and the piston seized in the bore. Both brand new proportioning valves. Were junk. If the proportioning valve does turn out to be your problem might not be a bad idea to reinstall the original proportioning valve after a thorough cleaning. I can’t say I’ve ever had an original that went bad.

Once you have good flow through the system, bench bleed your master cylinder again , Reinstall, and with the rest of the system ready to go do a simple gravity bleed. Make sure the master cylinder reservoir is full and simply open the right rear bleeder valve and leave it open until you have solid flow With no air just by gravity. Switch to the left bleeder when you are done with the right. This can take a while but it is the best way to keep air out of the system and not aerate the fluid by pumping the brake pedal. Nine times out of 10 you can bleed the system this way and you won’t need to touch it again.


Steve weim55 Colorado