[dana44]

Start by fixing the location you clamped, it may be the source of the problem. With the power brakes, it takes some pressure to push the pedal when the engine is off. You need an assistant so you can press the brake hard, have them open the bleeder valve, close it, then take another step on the brake. When lines and stuff are replaced, it can take several dozen pushes to compress the air in the line enough that the fluid starts moving, have had this happen to me before.

 

[ImperialCrown]

Or the shuttle valve in the combination valve is forced over to one side to allow only the front brakes to operate. This closes off fluid flow to the rear for safety and turns on the red dash BRAKE light. In order to center the shuttle, you need to open a front bleeder and push the brakes to the floor. Sometimes you will hear a thunk or pop and the red light will go out. Then resume bleeding.

 

[jamescurtis]

The shuttle valve isn't something I've come across before. I've done lots of brake work but never on an old fashioned full size domestic.

So either front brake bleed will work?

I'll check for a brake light on. I was letting rtv sealant on the transmisson pan cure so I didn't start the engine because the transmission is dry. Guess I'll start by filling the transmission and cranking the engine to get the light.

 

[ImperialCrown]

You should only need the ign key on. You don't need to start it.
Many older vehicles had the red BRAKE light illuminate during cranking only as a bulb check.

 

[jamescurtis]

Brake light was on when I checked today. But also, my son was able to pull the key out without turning it back to off. When I finally tried to start it after several hours of work it was dead. I had to boost it with another car.

No luck so far. I bled both front brakes. Ran easily.

No change in the brake light and no sound of a shifting shutter valve.

Then I changed out the flex line that might have been at fault. Of course the fitting broke so I had to flare the line and put on the new flex hose.

Then I boosted the van and ran it for a bit. Still no luck with bleeding the back brakes. Which was weird because the whole time I was flaring the end it was dripping down all over me. I had to keep stopping to top up the reservoir.

Does the shutter valve need power to shift back to centre? Should I bleed a front brake again?

At this point I'm really stumped.

thx for any suggestions.

 

[ImperialCrown]

The shuttle (pressure differential valve) works on the pressure difference between front and rear brake hydraulics. If a leak develops on one, it shift over to shut off the leaking side and turn on the warning light.
If you close the rear brake bleeders and open the fronts then pump up the brakes, the shuttle should be forced over. You may hear it pop and the light should go out.
All four wheels should bleed normally at that point.
brake-combo-valve.png

 

[jamescurtis]

Thanks for the drawing Imperial Crown. I've done lots of brake work on previous vehicles but I've never come across this problem. Do you know for certain that the 89 B250 van has this shuttle valve? The van is new to me but the symptom of brake light on and no ability to bleed the rear brakes fits the diagnosis.

The one strange thing was that I got a rapid drip while changing the rear line indicating fluid could drain from the reservoir but when I pushed on the brake pedal virtually no fluid came out the rear bleed nipple. Does this fit with the shuttle valve being out of place?

So, if I open up both front brake bleed nipples perhaps there will be enough pressure released to reset the valve? I'm not sure why it didn't work yesterday but I assume it needs a pressure difference between front and back and because I have air in the rear lines it didn't get the pressure it wants.

Is there any other way to manually reset these valves?

Are they prone to get stuck in the position mine seems to be in? I'd rather not have to replace it but if I can't get it to toggle back that's my next step I guess.

 

[jamescurtis]

Thanks to Imperial Crown's detailed advice I was able to put out the brake light.

At least temporarily.

I started bleeding the back brakes and found that if I pressed even slightly too hard it came back on again. Three times I returned to the front brake to put out the light.

Finally I decided to wait until I can get a second person to help me out. I'm used to just pumping away on the brake pedal and filling my one man brake cup until no air bubbles can be seen.

I've never dealt with a brake pressure differential valve, and although I can see the sense of them, if mine is typical, I'm glad they're not still put in new cars.

Thanks again Imperial Crown. You helped put me back on the path towards getting this old girl on the road, and that road will hopefully include Route 66 this summer if all goes well.

 

[Bob Lincoln]

You can't bleed brakes alone like that. Someone needs to hold the pedal down with the bleeder screw closed, then the other person opens it, allows the air out, closes it, and has the 'driver' let up on the pedal. Repeat over and over until air is gone.

If you try this by yourself, when you let up on the pedal to return to the wheel, the system sucks brake fluid back in, but inevitably will also suck some air in where the bleeder hose attaches to the screw. Also, you are pulling dirty fluid back in.

 

[jamescurtis]

Another couple of hours spent on the brake problem, with a mechanic to help this time, has nailed down the problem to the combination valve.

When the outlet connector was opened to the front brakes we got flow, but when the rear one was opened we got virtually nothing. When we tested fluid into the valve it was flowing easily from front and back parts of the master cylinder into the proportioning/combination valve assembly.

We've backed off all the lines in and out at one time or another and it seems that the valve is receiving fluid but still not letting anything through to the rear brakes. The brake light comes on when the fronts are closed and a rear bleed nipple is open if I push really hard, but usually it goes off again when I let up on the pedal. I've yet to hear anything audible such as Imperial Crown wrote in his post.

We even bled the master cylinder to be safe.

We tried all combinations of brake pumping with front wheel cylinder open, and tested the rears various ways but still only a trickle of fluid leaves either rear bleed nipple.

Do combination valves tend to fail or is it very rare?

I'm trying to decide whether to take it apart on the bench to see what's going on inside or whether to just start hunting down a replacement.

Thanks for any comments.

 

[Bob Lincoln]

Try opening all 4 bleeder screws and let them ooze. Gently push on the brake pedal with all four open. Gently. Then look to see which ones squirted fluid.

Then repeat what IC said, maybe stomp a little with the front bleeders open.

 

[jamescurtis]

Thanks to everyone for their help.

I bought a proportioning (combination) valve from a local scrap yard and was lucky enough to be able to install it without having to replace any lines.

After that the bleeding was as easy as every other vehicle I've owned. I guess something went wrong in the unit.

This problem's now solved. Stay tuned for other questions.