combo prop valve leaking hmm

[fast68chevy]

ok well the centering valve push end of my combo prop valve on the 75 C20 is leaking pretty good i have found out.. geting air in front brakes over and over and losing fluid over time.. this thing has been leaking for many many months now and i was unsure where how i was miysteriously and randomly losing fluid over time, sometimes overnight enough to have no brakes the next day and a bunch of air in the front lines calipers.
and now i finally know whats happened and has been going on all this time,,, the push button end of the prop valve came off,, the little round rubber boot, and is laying on the valve/lines crossmember and it is wet alot fluid from this end of it...


this now finally has gotten very worse,,, to where it is definitely and obviously leaking fluid failure from the combo valve.. so somehow something has gone wrong with it....
and ive just NEVER everrr seen or heard of this happening with one of these valves, over alll these tons of years now, that I've owned and driven and worked on tons and billions of trucks etc over the decades... and i was at a total loss and so confused couldnt figure out wut the hell was going on why i kept losing the fluid and totally losing the front brakes hydraulic pressure action, etc.. new master cylinder and no fluid in the power vacuum booster vacuum hose,,, and no caliper issues etc etc... totallyu had no clue and no exdplanation for any of it at al, and it was reallyyy starting to tick me off...

so... what could have possibly/probly happened to have caused this to happen with the valve,,, on this truck??
i have another 73-80 style valve somewhere stashed here i need to find so i can swap it out hmm...

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[Grit dog]

What could have happened? Idk, it’s a 45 year old truck and likely the original valve. Best part is it’s a cheap easy fix. Get a new valve slap it in, bleed the brakes.
I wouldn’t swap one 40 year old part for another 40 year old part if you actually drive the truck. Would seem your time is worth more than the cost of a new valve vs potentially having to do it again using old parts.

How did it take so long to find the leak?

 

[Nonstop]

Be careful of the new valves. I have replaced 2 or 3 of the aftermarket ones because of the same thing (too common of a problem). They all look like they are from the same supplier. The best one I found was from Wilwood. A little more money, but American made and tested per their site.

 

[Curt]

I got one from summit,Summit brand.Has been trouble free for 3 months and going.70$

 

[fast68chevy]

I got one from summit,Summit brand.Has been trouble free for 3 months and going.70$

hmmm yeah but what year style one.. an 80s one? because an 80s one is not same. the 70's ones were old iron steel early style. and i already have good '70s one here to use that i salvaged off a 1977 1.5 ton motorhome that is identical

 

[fast68chevy]

Be careful of the new valves. I have replaced 2 or 3 of the aftermarket ones because of the same thing (too common of a problem). They all look like they are from the same supplier. The best one I found was from Wilwood. A little more money, but American made and tested per their site.

yeah im not going with a new one, especially nowadays with all the reboxing of parts you never know what youre really getting,,

oh, and ijust now found this forum posting/info hmm. interesting...

https://www.chevelles.com/threads/fluid-leak-from-rubber-cap-on-proportioning-valve.633538/

 

[fast68chevy]

What could have happened? Idk, it’s a 45 year old truck and likely the original valve. Best part is it’s a cheap easy fix. Get a new valve slap it in, bleed the brakes.
I wouldn’t swap one 40 year old part for another 40 year old part if you actually drive the truck. Would seem your time is worth more than the cost of a new valve vs potentially having to do it again using old parts.

How did it take so long to find the leak?

nothing wrong with a 40 year old part if its in good shape, everyone has their own opinion about running old original parts if in good shape, and i swap on and run old original good parts all the time, and have tons of times over the past decades without any isue at all. ever.
this is really not a part that is very well known for failing one leaking... so very rare IMHO,,
this will be the very first time ive ever come across a failed/leaking combo valve, now,, and ive literally had or owned near 100 over the past almost 30 years now. hmm

and this is most of the reaon why i didnt figure this out alot sooner, because i literally did not supect the combo valve assy. until i finally noticed fluid leaking from somewhere up front that i couldnt explain.,

 

[Nonstop]

Be careful of the new valves. I have replaced 2 or 3 of the aftermarket ones because of the same thing (too common of a problem). They all look like they are from the same supplier. The best one I found was from Wilwood. A little more money, but American made and tested per their site.

I made this post yesterday. What do you know, This morning, helping out a friend, and his project at is up on the lift. There is a puddle under the car. Yup, aftermarket combination valve leaking. Another one to add to the failed list. The valve came in an aftermarket kit from a bigger company.

 

[ali_c20]

Where exactly is the valve leaking?

 

[Ken B]

Interesting posts. I am replacing brake lines and cant get the main line to the back brakes to break loose at the valve, I replaced all the rubber lines and steel lines..... I may let this sleeping dog lie, rather than potentially creating another problem to fix.
I do have another brake question , can you replace rear drum wheel cylinders without a complete stripping of the brakes?

 

[fast68chevy]

alright i finallly just found this brake combo valve assy i had wrapped up in a plastic bag and stashed put away,,, i really hid it from myself big time.. wow...

but now i finally have found it wow... so now maybe i finallyyyyy can now swap out/replace my blown out/leaking one,,, with this combo valve that i had put away well over a year ago now,,,,,
maybe finally sometime, in the VERYYY near future now.......
hmmmmmmmmm,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, and sure the two mounting bolt holes locations are in different places lot.. but that reallly no big deal at allll.,..

and i also have a couple or few of the 1980s years aluminum-style combo/prop valv asemblies, from half ton trucks and etc,, but they are actually quite a bit different plumbing And etc. and i reallly cant run those units, on my rig,, hmmmm,,, i included a pic of one of these 1980s 1/2 ton combo valves in the pics in this thread also.. that i also have in my parts stash here..

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[Goldie Driver]

Interesting posts. I am replacing brake lines and cant get the main line to the back brakes to break loose at the valve, I replaced all the rubber lines and steel lines..... I may let this sleeping dog lie, rather than potentially creating another problem to fix.
I do have another brake question , can you replace rear drum wheel cylinders without a complete stripping of the brakes?

1/2 ton (non full floating axle) yes, you just need to unhook one brake spring.

Can't answer on a fully floating axle.

Britt

 

[Ken B]

Thanks, I may change out the rear brakes to disc at some time but lots of things to restore and replace. dont want to dismantle the rear brakes , just need to do wheel cylinders to bleed.

 

[fast68chevy]

I do have another brake question , can you replace rear drum wheel cylinders without a complete stripping of the brakes?

yes you can swap wheel brake cylinders without pulling axle shafts, atleast on a semi-floater(half ton) or on a 9.5" 3/4 ton axle(these were under many 1981-up C/K 20 trucks)
but with a full floater 10/5" or any dana 8 lug axle with the bolted in in place axle shafts(eight bolts style full floater hubs) you will have to unbolt and puill shafts out and pull the brake drums/hubs off and so on..

the dana 30 and 44 axles are semi-floaters same as the half ton 12 and 10 bolt axles
- the axle shafts can stay in place when replacing the wheel cyls., just like with an ordinary GM half ton axle and a 9.5" axle etc.. as already stated/mentioned, above.


however, to replace a brake wheel cyl., on any axle model, you will have to disassemble the brakes enough to pull shoes back off/away from the wheel cyl ends, to be able to replace the cylinder.
which means pulling/removing the retainer pins hold down springs and pin retainers, and also the brake shoe return springs that pull and keep the brake shoes inwards-towards each other.. towards, and up against the ends of the wheel cylinders.

if your cylinders are actually still good, are not leaking any flud at all, and are working good/fine and you just need to bleed air out, then you can do it even with bad/stuck/frozen/stripped/damaged bleeder screws if you can soak and loosen up the hard lines a few threads and then push pedal down then tighten hard line back up before allowing the brake pedal to return back up.
and do this a few times to get all air out.. is that all you need to do? its not the "correct" way to bleed the rears.
but it will work if the bleeder screws wont budge or whatever.
this is what I have had to do many times over the years, in many/several instances, and with many random/different vehicles.
and I have also had to bleed some air out of the front lines/hoses by loosening the brake rubber flex line banjo bolts too.. and this has worked fine also.. just to bleed out air from lines/hoses.. quite a few times over the years, too..

you cant get the bleeder screws loose, or cant remove them to replace them, or?

i have included a pic of a complete rear axle brake backing plate assy with the brake shoes and everything all intact, from a 1/2 ton 8.5" axle i stripped/parted out last year,, with the larger 2-3/4" brakes so you can kind of see what im talking about., how the brake shoes are pulled together into both ends sides of the wheel cylinders, and why you have to pull the pin retainers and return springs in order to pull shoes apart in order to get the whee cylinders off and out, from inbetween the shoes.. its upside down in the pic.. but you may be able to get the idea possibly maybe.. if not i can get alot better of a pic or two,.,

i always pull and save thesde wider 2-3/4" backing plate brake assmblies because they are perfect for swapping onto 1969 and older 12 bolt axles to make them 5 lug with stock length axle shafts form any 1971-1987 axle. withotui the need to modify anything. beause of axle width change in 1970s wider by 3/4 each side,. which is 1.5" wider axle total, in which is made up for by the wider 2-3/4" brakes, perfectly. and this also swaps them from 6 studs to 5 studs at the same time..
this is for 1969 and older only though, because these were a 1.5" narrower axle.

1970 is the oddball 1.5" wider but with the old 6 stud axle shaft and drums still.
but for a 1970 model year axle all you need are a pair of stock 5 lug axle shafts from any 1971-87 axle ande you dont need to swap on 2-3" backing plates and shoes in order to do the 5 studs swap-over/conversion. but then i dont even mess with half ton trucks anymore.. because i really like 8 lug especially dually trucks alot more nowadays and have for a long time now the half ton trucks etc. were in my earlier years kind of a thing... like my very first truck,, the 1968 C10 #1 in 1997(i have a huge monster photo album just full of pics of basically all the trucks and vehicles and etc, that ive had, and dragged in here over the decades...)

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[fast68chevy]

Where exactly is the valve leaking?

right from/out of the end of the main valve/piston,. wher the rubber cap button end is,.

it actually blew the button rubber cap off the end of the combo valve due to fluid leaking out under pressure.. off the end of the combo-valve assy.
and the rubber cap ended up landing and staying on the same very front "V-shaped" middle-bent crossmember that the brake combo-valve assy is bolted to. and quite a bit of fluid on it too.. in which has been coming out of the rubber cap button end of the valve assy... so its blown/shot.

and this is a first for me, even after all of these decades of owning/building/parting/selling/trading and driving these and also much older GM and other trucks too,, I can say that I never once have ever seen/heard of one of these valves blowing out/going bad/failing like this, ever, ever, ever, everrr..........
i just never knew or thought this was even a possibility, with one of these valves, wow..... im still kind of in awe or shock, and am basically sort of surprised by this occurance,,,

i have a good valve assy from this 14K GVWR "1-1/2 ton" motorhome 1977 model year, sitting here and ready, that i am gonna swap on and run.,. it is about 90 percent identical, the only real actually difference i have to deal with to swap it on and run it, is to swap around the two line from the m/c as they just have reversed locations on the top side of the valve assy., and thats all.. but this just an easy/minor thing.. no biggie at all..