bannerd

Hey all, coming back from work I lost my brakes. My combination valve is not playing nice, I'm assuming this is where the problem is. If I hit the brakes, fluid will shoot out of the master cylinder and almost hit the hood. Usually it just bubbles out. Think I may have air in the line (I read through so many brake threads on this form) I bled them and had my wife help me. There is no clamping force at all, I can watch the caliper sort of push the piston out but no force. I'm going to bypass the combination valve tonight and see what happens. What is the purpose of this device?

bannerd

Sounds like I will need that valve, at least to prevent the rear from locking up. I was looking at getting one of these;

http://www.jegs.com/p/JEGS-Performan...55192/10002/-1

Run the rear to that and run it low while keeping the load to the front. What do you all think?

j_martin

Master cylinder internal damage. I don't see any way the combination valve could this.

I'm assuming when you push pedal down, it sprays back. If the spray occurs when you release the pedal, it could be air. If it's air, you should be able to get pedal by rapidly pumping it. The sudden ingestion of a lot of air without an empty master cylinder would be really rare.

The combination valve holds off the front brakes just a bit while the long rear brake circuit gets it's pressure up. It also senses if one circuit is flat, and lights the brake (fault) lamp. If there's air in the lines somewhere, it should light the lamp.

mknittle

Yeap master cylinder. The seals are bad are bypassing back into the reservoir.

bannerd

If I push the pedal, it will spray right out of the master cylinder. I don't think the master cylinder is damaged. If I have my wife pump and hold the pedal, then I loosen the brake bleed it squirts out of the bleeding nipple with a lot of force. If she pumped the brakes the pedal gets firm but then slowly goes back to the floor. The combination valve might be done for. My plan was to hook the front up and bypass the combination valve. If that works then I know something was wrong with it.

I think the brake fluid is somehow mixing with the front/rear at the same time causing some strange things to happen.

Angry Johnny

The combination valve simply restricts pressure to the rear brakes so they don't lock up first. There is inside a switch valve so that if there isn't enough pressure on one side it will turn the warning lamp on.

Since you have pressure at the bleeders I'm thinking you have some near frozen calipers or flex lines that have come apart inside. Not uncommon on old hoses.

Now if the pedal will go all the way to the floor than you either have a leak or a bad master cylinder.

j_martin

What's with the combination valve fetish? Even if what you think is happening were happening, it wouldn't make any difference as both hydraulic circuits run at the same pressure.

Bleed the back brakes. If you get pedal, a wheel cylinder is letting in air. Don't know exactly how it happens, but it does.

If you don't find a lot of air, and don't get pedal, (expected result) replace the master cylinder. One of the 2 piston cups in it is blown.

If a caliper were frozen, you'd get a hard pull when braking and have full pedal. Same with a blown rubber line. Not likely both sides would fail the same way at the same time.

I repeat, most likely the master cylinder is damaged. Bendix units still available on Amazon.

bannerd

Might be the master cylinder, everything on there is new and I went all bendix. Calipers, master cylinder and back brakes are all bendix parts / pads. I went through it when I was running the nickel/copper brake line at the time. I still have the old master OEM cylinder, I will throw that on tonight and see what happens.

Thanks guys!

patdaly

Yep, unfortunately, even high quality parts have an excessive defect rate these days.

Still good to go with the best you can.

I second and third what the others have said, MC.

bannerd

I point my finger at the combination valve because my rear tires lock up if I brake moderate-hard. The original one was rusted out and looked like a dog turd when I removed it. After finding one at a junk yard it has worked for awhile but I think the new brake system blew the o-rings out of it. The MC is most probably gone, still under warranty as it's only a few months old I would say.

Has anyone ran adjustable valves? Is it stupid to run 100% power to the front and then do a 60% decrease for the back drums?

mknittle

Every time I have seen this happen It is the master cylinder. what happens is the rest of the system holds pressure. The bad seals are the weak link in the chain. So that is where the pressure goes. back into the reservoir.

j_martin

This combination valve holds off the front, not the back like a race car. If the stem sticking out of the front of the valve moves when the brakes are operated, the valve is functioning correctly.

If you use the jegs one, you'd put it in the front circuit. Losing the combination valve also loses the warning light that one brake circuit is failing.

I don't think the combination valve is the problem.

Rear wheel lockup is a function of ABS failure, or non existence. The big drum brakes tend to get a bit sticky under some weather circumstances anyway. I always plan on one surprise by the end of the driveway, and drag the brakes for a few hundred feet when I get to the pavement to dry them out and normalize them.

Sudden failure of rubber parts is often caused by the wrong brake fluid. I've seen it when DW put power steering fluid in the master cylinder. I've also seen it when my kid put DOT5 in a DOT3 motor home system thinking it was an upgrade. If you accidentally did that, you might as well just replace everything with rubber in it after flushing the steel lines.

bannerd

I should have explained more history about my brakes. The ABS unit was removed a few years ago. It worked, but for some reason it was kicking on all the time during the winter causing my rear to slide all over the place. It was actually really dangerous for me. The light on the dash was really annoying so I pulled the bulb and the brakes on the truck worked awesome... until salt and road materials did some major damage to the entire brake system. I replaced all the lines on the truck as I had some serious leaks. I noticed my calipers lost the rubber seal around the piston and low and behold it rusted the piston stuck. 500 dollars later everything was working, I took the ABS modulator out and I left the combination valve in place. The truck was braking like a dream. I will get the master cylinder replaced and just hook up the combo valve and see if that is working. I was looking at the rod last night and I could move it in/out with my finger. There is a small rubber seal around it.

j_martin

The RWABS on these things is pretty primitive to start with. The lock up you were experiencing, however, was just sticky brakes. The rear 3" brakes driven by 1 1/8" cylinders are pretty aggressive to start with, and work well with about 6000 lbs on the axles. They will get sticky, and with little in the cargo box, they'll lock up on ya, especially at the start of the day.

I just got Blue (D350) back on the road last night after the multi-round NAPA brake booster saga. Been a couple of weeks since it has run. Booster at full performance, and I was quickly sideways on the wet road with the first stop. Worked fine after that.

The reason they can be sticky is that the design is such that the forward rotation of the drum adds to the braking force by levering the shoes. That design can cascade to lock up if the shoes don't readily release. Good shoes, nice clean smooth drum, no leaks, and don't get them wet, and they are perfect. A very small wheel cylinder leak will cause them to lock up sometimes.

If you look at that rod when someone else is stepping on the brake, you'll see it move a bit. If it ain't broke, don't fix it.

thrashingcows

I had a MC on the crewcab go bad. The truck still stopped fine, but the front reservoir would empty into the rear one. I'd open the cap and fluid would pour out. Sounds similar to your issue.

I have run the adjustable proportioning valves before. I actually run one on my 68 T/C wagon. I run straight off the MC to the front brake lines, and the adjustable valve to the rear. Takes a bit of fine tuning to get the rears set-up right....but the system works great.