No fluid to rear drums: WTF?!?!

[BigTopGT]

I recently picked up a decent 71 C10 (LWB, 350/350 auto) with a soft brake pedal.

I got her home, replaced the booster, the master, and the Proportioning Valve, then tried to bleed. (the master ws bench bled)

After several days of messing with it, I just CAN NOT get fluid to the back brakes. I even went so far as to take it to a local Meineke and had them try to use their 3 bar power bleeder, but THEY just called and told me they can't get fluid to the rear, either.

I just don't get it.

I mean, it's not a complicated system, and I replaced everything except for front calipers and rear wheel cylinders, so what the eff can this be?

One thing of note: if you crack the rear line at the bac of the Proportioning Valve, yoh don't get any fluid, but I'd you crack it at the master, it leaks.

Could I simply have a brand new, but still defective, proportioning valve?

I'm at the end of my rope here, guys... Someone is coming to pick up my 67 today and I need a truck to drive. :/

[Richard2112]

Proportioning valves are designed to shut off the rear circuit in the case of low pedal pressure. When this happens it's necessary to reset the prop valve. This can sometimes be done by bleeding the fronts and once done, delivering a hard stomp on the brake pedal.

The proportioning valve is designed to limit pressure to the rear circuit in the event of "panic braking" so the rear drums don't lock up. Outside of a "panic brake" event, the prop valve shouldn't do anything as the front and rear pressures are determined by the MC.

More detailed explanation....https://itstillruns.com/proportionin...-12040941.html

[Tupper81]

did you bleed the proportioning valve?

[jocko]

The main function of these valves (prop, combo, distrib blocks, etc) is to ensure isolation of the circuits if one of them loses pressure - so that a leak in one will only results in you being left with front or rear, but not neither. Some prop valves are adjustable so that you can bias and/or balance the braking system. Different valves are used for different applications - i.e. 4 wheel disk vs disk/drum vs drum/drum.

Yes, can you indeed have a bad, brand new prop valve. Where did you get it and what brand is it?

Was the original distribution block on there before? They are very reliable, non-adjustable, and well suited for these trucks. If it's in the trash, go grab it, they are hard to find and expensive. I'd reinstall it if all else fails.
Prop valves can be reset as Richard mentioned - this recenters the shuttle valve between the circuits. (do you have the wire connected to the block that triggers your dash light, and, if so, is the brake light on on you gauges?)
Also, if the valve is adjustable, it may be totally biased to the front or something odd like that - but I kinda doubt that's the issue.
Again, not at all unheard of for aftermarket valves to be bad out of the box.

[bigmac73]

i thought there was a GM tool used to jack the prop valve open a little clip type that went on the front of the prop valve

[BigTopGT]

Originally Posted by Richard2112

Proportioning valves are designed to shut off the rear circuit in the case of low pedal pressure. When this happens it's necessary to reset the prop valve. This can sometimes be done by bleeding the fronts and once done, delivering a hard stomp on the brake pedal.

The proportioning valve is designed to limit pressure to the rear circuit in the event of "panic braking" so the rear drums don't lock up. Outside of a "panic brake" event, the prop valve shouldn't do anything as the front and rear pressures are determined by the MC.

More detailed explanation....https://itstillruns.com/proportionin...-12040941.html

That's the struggle: it seems the Proportioning Valve is brand new, but behaving as if the prop valve has been Tripped.

[bigmac73]

this may help

[Richard2112]

Originally Posted by BigTopGT

That's the struggle: it seems the Proportioning Valve is brand new, but behaving as if the prop valve has been Tripped.

It can be tripped just by bleeding the brakes. The prop valve senses an open circuit and shuts down the rears. There is a little plastic stopper sold which prevents that occurrence while bleeding. I'll see if I can find a link...

Yes, it's the tool posted above by bigmac73

[jonmc56]

I found this video helpful in understanding what happens when the prop valve trips, starting at about the 3 minute mark in the vid.
https://youtu.be/xrjENVu1Bxk

This didn't turn out to be my problem, in my case I couldn't get fluid to the front circuit because the rod between the master and the booster was slightly too long (holding the master cyl piston in a position where it couldn't get fluid). It doesn't sound like this is your issue, hope the video above helps you.

[jocko]

Originally Posted by bigmac73

i thought there was a GM tool used to jack the prop valve open a little clip type that went on the front of the prop valve

There is a specific tool for the original GM blocks - looks like OP has an aftermarket mc, so not certain which tool would work, the original might. Panic brake stomp is the backup method to re-center the shuttle.

[HO455]

It is quite likely your new proportioning valve is bad. I'd wager there is a bit of machining gone wrong inside. Time to call the company you got it from and ask for a refund.
As previously stated there really isn't many reasons to replace a proportioning valve. When you go get a complete brake service at a brake shop they don't replace them. They replace everything thing else that could conceivably fail out of liability concerns. Proportioning valves are extremely reliable, especially for simple ones like ours.
I would reinstall the old one and see if you can bleed the rears with it. Then I would return the new one and never look back.

[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

[Happy_dan]

Steve, that is a great writeup. Thanks for taking the time.

[BigTopGT]

Circling back to update with the solution. (because I hate when people do that)

It turned out nobody checked the lines and when the previous owner lowered it, nobody realized it was linking the lines.

When it was on the lift and the suspension was at max travel, everything was fine, but when it was on the ground, especially when the bed had even a LITTLE weight in it, the soft rear lines kinked something awful.

They'd let fluid in the first time you hit the brakes, but the fluid couldn't get back out, so it held them.

I realized what it was after trying a bunch of other diagnostic methods, but a simple unbending the line with the truck on the ground resolved the issue immediately and entirely.

Thanks for all the advice, folks.

[ghackett1]

Check the rear flex line that goes to the “T” above the rear end. If that fails = no rear fluid/brakes.

[Stocker]

Glad you were able to find & fix the problem, and thanks for coming back to post what you found!

[HO455]

What he said!

[special-K]

I just got back online since Saturday morning. Read the first post and my thoughts went to collapsed brake house. I saw all that was new but no mention of brake hose. glad someone figured it out