[Shiny]

How should the pin on a brake combo valve behave? Mine is stuck "in" and I tried bleeding a front line as suggested on other threads, but it did not pop back out .

The valve (iron body used original 71 or 72) was taken apart, cleaned up, and put back together before installing, and I made sure the internals were free. I have a soft pedal, and have bled every wheel repeatedly, with a vacuum pump, with pressure-then-crack, etc., but still not firm. I expanded the back drums, thinking they may have been too loose, but still not right. I pulled the master and tipped it "down" and repeatedly pushed the piston to try and burp it, but no improvement. The vacuum pump had no trouble pulling fluid through all 4 bleeder valves.

How do I diagnose the combo valve? Is there a preferred sequence or method to get it centered?


thanks

[GStage1]

The relief plunger should come out when the master applies pressure to the combo valve...you can simulate this using compressed air....if the air works the relief plunger...you may have a weak/bad master....buy a new master...never a rebuilt one.

[Shiny]

GStage1 - thank you for the suggestion

which port would I pressurize?

[GStage1]

Use the front port from the master cyl...then place your finger over the two front ports...when the compressed air enters the master cyl port it should "pop" out the plunger....if not, you have to disassemble the combo valve and clean it...most likely you have some contamination in it.

[Shiny]

The central piston was "stuck", and I may have had the proportioning valve assembled incorrectly.

Are there any detailed diagrams of these valves available to guide a rebuild?

Can I get replacement cups and o-rings... basically a rebuild kit for these combo valves?

[GStage1]

There is a cut-away in the factory shop manual....no rebuild kit that I know of for them.
You can easily get another combo valve in case yours is not repairable.

[Shiny]

thanks again... I don't have a manual with that drawing, as my car is a 70 (separate metering and proportioning valves) but I used a combo valve from a later model.. I found a drawing on the web, but it is not very clear..if someone could scan and post a drawing from a 71 or 72 manual I would be grateful..

I may spend the money and get a replacement, but the valve I have seems to be in good shape internally.. the cups are still flexible and the bores are not pitted so I will try to use it one more time before giving up. I bench-bled the master, and did see a lot of bubbles, so maybe that was a part of my problem too.

The metering valve was free, so the fact that the pin would not push out must mean the pressure never built up enough in the rear circuit to pressurize the front. Does this make sense? I have new lines, new rubber hose in the back, rebuilt drum cylinders, and I adjusted the shoes manually.

Is it possible that a master with air in it would prevent the pressure from building enough to actuate the metering valve?

Does the central switch piston do anything more than actuate the switch with a differential pressure?

I suppose if my switch piston was stuck, I may not have had a good indication of which circuit was weak?

[GStage1]

Air will cause your problems.....assume you have a new master....takes several cycles to bench bleed them.

Sounds like your combo valve is ok.

I would try bleeding the system again.

From the factory shop manual, you should have the pin pushed in while bleeding the brakes.

[Shiny]

GStage1 - thanks again for your help and support.

I put the valve back together and will give it another shot.

The master was difficult to bleed. As you suggested, it took a lot of pumping before I stopped seeing air bubbles, maybe 20 cycles. After I got most of the air out, I noticed small bubbles every time the piston returned. I tightened the fitting on my bleeder set-up and this stopped, so apparently there is some suction on the fittings when the piston retracts. I thought there was supposed to be a check valve in the master ?? guess not... anyway, I hope I got it this time.

When I had the valve apart, it appears the pin should "normally" be retracted. It looked to me that the pin would be pushed out when pressure "opened" the metering valve. I think the idea with the clamp must actually be to prevent the pin from pushing out, which would keep the metering valve closed and allow pressure to build on the front circuit ?? If I am wrong about this, please let me know. I should probably just buy a new valve, but after all this effort, it would feel like I was rolling over !

I'll keep my fingers crossed and see how it goes this time.

[Shiny]

Broke a small cast iron disc (1/8" thick, looks like a miniature man-hole cover, maybe a flow control thing or just a spacer) in the pressure-differential part of the combo valve when I was dis-assembling. I put it together anyway, without this part, and installed the valve, bled again. Brakes were completely gone, worse than ever, so I blew it.

Know when to fold 'em.

I ordered a new brass repro valve and will check the master again... and hope I don't need to replace that too.

I figure I will plug the master outlet ports and make sure I have pedal.

Any better way to do this?

Thanks again.

[Shiny]

Installed new combo valve. Loosened fittings at master 1/2 turn, pumped pedal about 20 times to try and bleed master again. Seemed to be flowing fluid after a while with no air. Then bled at each wheel until no air with helper building pressure, opening, closing before pedal release, etc.

Still no pedal. When pedal is depressed, can see pin on combo valve protruding about 1/16 inch, so some pressure is building up, but still acts like air somewhere.

So do I have a bad master? There is no leakage at piston.

How do I tell if rod length on booster is correct? My master has the piston right at end of casting, with only a slight recess to retaining ring. My booster has a very short rod extending from what looks like a 3/4-inch dia cylinder. Is it possible the rod is too long? How do I tell?

When I push the pedal, it goes in about half travel with little resistance, then I can feel a clunk like something inside the master is engaged, then there is a little more resistance but still goes to the floor.

Is it possible for a master to be bad without leaking? How can I tell for sure before throwing more money at it?

thanks again

[GStage1]

New master cylinders with the shallow depression use the short power booster pin...sounds like you have the correct set-up.

From your testing, sounds like the master is bypassing internally and causing your problems. Try a new master cylinder.

[Shiny]

Someone please tell me what I am doing wrong..

I have not been able to get my brakes working and it is making me crazy.

I had a soft pedal, so after replacing the combo valve with a new one with little improvement I found a leak in the rear line, where I had tied into the existing line and made a bad flare. I fixed this and had no fluid leaking any more, so I thought I had it beat.

wrong

so I rolled over and bought a new repro master from Inline Tube and installed it

I just finished bleeding for the 2nd time and STILL have a soft pedal.

Here's where I have been, what do I try next??

bench-bled new master, no bubbles
bled master again in car, no bubbles
bled with a hand vacuum gadget at all wheels until no bubbles

after this, I had some brakes, but still soft... so I had my son push the pedal, and I bled all over again starting with master, but this time tightening the bleeders before retracting the pedal..

I did get bubbles at the rear, which I don't understand after using the vacuum pump to pull about 4 ounces of fluid through each side, but we pumped until I got 4 strokes with no bubbles at each cylinder

It is STILL NOT RIGHT... almost to the floor before I get anything

I backed up about 20 times, thinking maybe the rear was just loose but did not improve

I have been putting a clip over the pin on the combo valve to keep it from pushing out.. do I need to do something else with the pin ?? does the "factory tool" mentioned in my service manual actually push the pin into the valve or just keep it from pushing out?

I think I have pumped 2 quarts of fluid by now and am just maybe a little frustrated

I guess I will check the rear line again but I don't see any fluid leaking, so I don't know how air could be sucking back in..

Is it possible for air to be in the master but not be eliminated by bleeding in the car through the bleeders on the master? How would I know and how would I remove it?

I can see and hear the front calipers strain when my son pushes the pedal, so I think my problem is in the rear circuit still, but I thought the front and rear were independent, so would the front not build pressure if the rear had air? If the rear had air, and the front did not, would the switch activate on the combo valve? would it stay tripped or return with pedal pressure removed?

this is not supposed to be this difficult, is it?

[GStage1]

Front and rear circuits are separate.

Call InLine and ask them what length should the push rod be in the power booster.
You may have a rod that is too short.

Everything else you have done is correct.

[Shiny]

Any more advice on how to get my most frustrating brakes working? I am out of ideas and completely humiliated after fighting this on and off for over two months.

After installing a new booster and new combo valve with little improvement, I called Inline Tube for advice. Here's what they said and what I did, with no improvement:

- verify master is air-free by disconnecting lines, plugging at master, and making sure pedal is solid

- relieve possible pressure differential by bleeding very slowly through both front calipers simultaneously

- check for pressure differential by monitoring switch on combo valve while bleeding, center if switch actuates by bleeding the other (front or rear) system

- manually adjust rears until shoes are engaged, then back off slightly. They confirmed residual pressure should be maintained by the combo valve

- bleed and bleed and bleed, then let car sit for a couple days and bleed again

I just went through all this for about the 5th time, and it seemed almost good in the garage without the power boost. There were no bubbles anywhere, the pedal with engine off was almost firm, and I could see the front calipers squeezing, and verified the rear drums were braking. I held pressure on the pedal and it did not drop. So I got cocky and actually drove around the block and it quickly softened up again. It acts like air is getting in somewhere, but I can not find any fluid leaks. After about 10 stops, the pedal was at the floor again before I got any braking.

I am totally stumped (OK, it might not take much), and need more ideas before I rip the whole brake system apart again.

I will try bleeding again.. I suppose if I see bubbles, it will tell me where I might have air getting in.

[GStage1]

It has to be a bad new master....buy a new from Autozone....I believe it is TRW/Moog or Raybestos....runs about $42.

From your description, the master is not building pressure and it is internally by passing.

I assume you have verified all new parts are good....hoses, calipers, wheel cylinders...etc.

[Shiny]

Thanks for the suggestion. I have a new master from Inline Tube. They told me to plug it and check the pedal. When I did, the pedal was rock solid, but I will call them again and ask about your bypass idea. How do I verify rear cylinders are good? I have a posi rear that makes it hard to rotate rear drums, but they do seem to apply some braking. The front calipers strain when I get pedal, so I think they are working.

Going camping, then vacation, but when I return, I will bleed again. If I see bubbles, then get rid of them, and it happens again, I will start replacing more parts, I guess, probably starting with the rear cylinders. About the only thing that is not new is the front-to-rear hard line.

[Shiny]

Will someone please tell me how to figure this out? I am totally out of ideas but it has to be something obvious.

I am totally stumped and really need help diagnosing this. Please tell me what else I can do to find the cause of this.

I bleed until I don't see any air bubbles. The pedal feels solid, then after a few brake applications, the pedal is almost back at the floor before I get any brakes. I must have done this 10 times without seeing fluid leaking anywhere, but it always ends up feeling like air is getting back into the lines.

This last time I bled the system, I did see a few bubbles at the rear cylinders, but they were gone quickly (after about 2 pedal strokes) and the pedal felt firm and did not go to floor with constant pressure. I then verified the rears were getting pressure by trying to rotate drums with pedal pressure. The front calipers get pressure as I can see and hear them strain.

The master and combo valve are new from Inline Tube. They told me to verify the master is good by removing the lines and plugging the outlet ports, which I did, and it felt very solid. They then told me to check for imbalance in the combo valve, which I did, and have never seen the switch close. They told me to "reset" the balance on the combo valve by slowly bleeding the front circuit with both calipers open. I did this too, with no fix.

I checked the pedal and it seems to let the booster return OK. I can see and feel a little slop at the beginning of the pedal stroke.

How do I confirm the booster is letting the piston in the master cylinder return fully ?

Is there some way air can get back in without fluid leaking out ?

When the pedal goes to the floor, I can feel an initial resistance, then about half-way I feel a second resistance. Is this expected based on the master internals or does it suggest, as GStage1 said, that I have a bad master ? I think I read that the front and rear pistons are linked by a spring inside the master. Is this why I feel that mid-stroke change in resistance ?

Could I have a bad master and still have it feel solid when I plug the outlet ports (per Inline Tube advice) ?

Please please please (yes I am begging) tell me what can I do to diagnose and isolate the cause... I have spent way too much time on something so seemingly simple and am starting to not like it much.

[ELKHORNAOG7]

You may try to have the rear of the car jacked up in the air this way if there is any air in the line it will go to the highest point in the line then you can try to bleed any air by the 3way fitting at the rear diff. and work your way down to the wheel cyls.--- another thing you can try is if you know your front brakes are working try and put a plug in the front of master cly. where the line goes this way the front is cut off and just try bleeding the rear I don;t know if this will work or not but it is something different and it may work you tryed everything else....There is one more thing you can do you can call 888-351-8781 ( master power brakes ) and ask them to send you a ( catalog and technical manual ) this catalog has everything you want to know about brakes, like how to know the lenth of the rod from power booster to master cyl. this book is a must have when you work on your brake system. www.mpbrakes.com I hope this helps,---BOB

[ELKHORNAOG7]

I just checked on that www.mpbrakes.com and they have a tech section that you can go to and this may help you----BOB