Hello:

I have a 1976 F-250 (351M, 2WD) that had been sitting for a couple years, and I recently got it running again. Immediately I discovered two brake problems. There was a leak with the front driver's side line, and the rear passenger's side brake cylinder was also leaking (fluid coming from within the drum). So, I replaced the front driver's side line and bled it, everything was fine there. I replaced the rear cylinder and also replaced both rear lines from the Y connector to each side, because they were both in rough shape. Now, I cannot get the rear brakes to bleed. I've tried manually bleeding with somebody pushing on the pedal without the engine running, but nothing comes out of the bleeder screw on either side. I also started the truck and pressed the brake and still nothing.

Can anybody offer any suggestion as to how to get this to bleed the rear brakes? Thanks in advance!

 

first make sure the reservoir is full. then unhook the line going to the axle to make sure fluid pushes through that far. then (with both bleeders closed) start with the right rear and bleed normally. it may take a while to get fluid through the line. once you get fluid to the pass side, do the drivers side. you may have to go back (side to side) once or twice to get all the air out of the system after you get fluid to each brake. Just make sure the master cylinder stays full and keep trying. good luck!

 

i had this proplem on my 77 last week as well. it took me almost 20 times of having my wife pump the barkes and hold while i opened bleeder valve to get the fluid to make it thru. it maybe be you just have that much air in your lines. also make sure you are bleeding the lines correctly. if you have the bleeder valve open when someone lets off the brake pettle you will suck air back into the lines.

 

I'm guessing your proportioning valve is faulty.

 

I always start by gravity bleeding. Fill the resevoir then open the farthest bleeder (right rear). Keep the fluid level up and wait till it flows from the bleeder then close it and open the left one. It takes a while but it always works for me as long as the lines are clear. I usually pressure bleed it a couple times after just to be sure.

 

there has to be soething blocking flow to the rear lines, or a bad proportioning valve like highboy said. i have done tons of breaks on all types of vehicles, and even with new lines from the master cylinder back to the rear wheel cylinders, there isn't enough air to keep you pumping for more than a minute. did you leave the cap off of the master cylinder to let the fluid flow? i would fill the resivoir in the morning, leave the cap off, and crack the rear bleeder on that side. check it periodically, and i'm sure some fluid will come out

 

^^ I agree, and I'll add if you did let the master cylinder go empty you will probably need to bleed it as well.

 

Thank you for all of the feedback. Today I took a closer look and it appears that no fluid is coming out of the pressure differential valve that runs to the rear brakes. I made sure that I pulled the bleeder rod out on the pressure differential valve, but that did not help. But, I noticed that when somebody presses on the brake (truck not running), that the bleeder rod pops out. Is that supposed to happen? Can this valve be "reset" or is it pretty much shot? Thanks again, thankfully I am getting close to getting this resolved as the issue seems to be the valve for sure.

 

OK, well I got a replacement proportioning valve (used) from a salvage yard. But, I'm having the exact same problem! Here's the current status:

I hooked up the replacement valve to the old lines, and immediately had somebody press on the brakes and fluid came out from both lines from the master cylinder. Then I tightened the lines. But, I'm having the same problem as before. No fluid is coming out of the proportioning valve to the rear line. I don't get it. Is it possible that the master cylinder is the problem? That's the only thing I have not replaced yet. I didn't think it was the master cylinder as fluid comes out of it for the rear line to the valve, but no fluid seems to be making it through the proportioning valve. Any help would be greatly appreciated. Thank you!!!

 

This happens when the proportioning valve shuts off flow to the front or back brakes under a catastrophic brake failure (loss of pressure due to fluid leak). Try closing all 4 bleeders, then climb in the cab and slowly press the pedal down several times in a row. Hopefully this will re-center the valve, such that fluid can go to all four wheels instead of just the fronts.

 

Had the same issue with mine. The prop valve had stuck in the safety position. I took my valve apart, blew air through it, tried every thing I could think of... NO LUCK... ended up going to the local salvage yard and grabbing three prop valves.

I bench tested one with compressed air and it blew clear all ports. Installed in truck and ten minutes later I had all the brakes bled!

Took a few weeks to straighten out, but things look good now thanks to the folks on here!

 

fmc400, I did just as you said and still no luck. In fact I did that to both valves, and still no luck.

dynamic, that's a great idea to push some air through. I will probably try that just to see if this replacement valve has anything going through. How did you manage to find fittings that worked in all of the entry points of the valve?

Thanks!

 

[QUOTE=szilagyic;8940209]fmc400, I did just as you said and still no luck. In fact I did that to both valves, and still no luck.

I've seen this problem before and it kicked my butt years ago. On the back of the PV, there should be a rubber boot. Inside this boot is a pin. No need to remove the rubber boot, use a pair of needle nose pliers and squeeze the pin through the boot and pull the pin to the rear (out) while someone pumps the brake pedal. This of course, assumes the the MC is full of fluid, the brake lines are connected, etc.

 

No fittings necessary! just get out you blow gun with the tire inflator (needle looking) fitting, or the rubber tip end and blow some air in. Keep in mind that depris and fluid should be coming from the other end. Keep your open cuts away from the brake fluid under pressure!

 

Thanks dynamic. OK, finally got the problem solved. I ended up finding a new valve on Ebay, and sure enough it works fine. All inlets/outlets on the valve work now. Got the brakes bled and all is well. Only took about 6 pumps and fluid came out of the rear cylinder(s).

Just for the record, before I installed the new valve I was able to take my air compressor and blow air through the front/rear inlets, and air came out of the outlets just as dynamic mentioned. I also never messed with the bleeder pin that everybody has been talking about, I bled both the front/rear without using the bleeder pin.

Thanks for all of the help!