Adjustable prop. valve ?

[Jonboy]

I am looking for some help selecting an adjustable prop valve for my truck. It is a '74 Crew cab (originally 3/4 ton). I am using the 3/4 ton master cylinder and booster (JB1, IIRC), D52 front calipers (already on the truck, 3/4 ton spec), and a 1/2 ton 12 bolt with drum brakes. I am having problems getting the original prop valve to recenter, so there is no fluid going to the rear brakes. I may swap to rear discs in the future, but probably not at this time. The 3/4 ton master has equal size fluid reservoirs, so I am guessing that the fluid volume front/rear is the same as well. Any suggestions to which prop valve to get (ideally adjustable), or any other problems with what I have?

[Keith Seymore]

A few observations:

a) Probably not JB1, since JB1 was the smallest manual system available. JB6 or JB7, depending on GVW, I would think. I happen to have the brake design/release charts for 1974, so you can see how it was originally released.

b) Are you suggesting you would remove the production prop/combo valve and replace with an adjustable valve, in the rear system? That would probably be ok.

c) They should be available through Summit and Jegs, but also the shops that cater to roundy-round guys (like Speedway, if it still exists).

K

[ERASER5]

Make sure you retain the 10# residual valve. It is usually in the MC rear brake port. Also, now that I think of it, make sure you are hooked up the correct MC ports. The routing for a 3/4 is the opposite of a 1/2. Swapping ports would likely cause issues as the residual valve would be holding the pressure on the wrong brake set. Wouldn't it have been great if GM had just done them ALL the same? I know... crazy talk.

[Keith Seymore]

Originally Posted by ERASER5

Wouldn't it have been great if GM had just done them ALL the same? I know... crazy talk.

Well - I've thought about that.

I think originally they were front reservoir went to the front brakes; rear to the rear - which is intuitive.

However, as hood lines got lower they moved the smaller reservoir (rear drum) to the front of the master cylinder to accomodate the slope. I had an L van (Astro/Safari) aluminum/plastic master cylinder on my Chevelle and had to jack the rear end of the car up just to put fluid in it because the front reservoir/rear system was so small.

By the time you look at all the parameters (bore diameter, quick take up vs non, English vs ISO ports, inboard vs outboard ports, reservoir size and the aforementioned location, etc) there are a million ways for a master cylinder to not fit. I was blown away, even as a GM brake development engineer.

K

[ERASER5]

I guess I wasn't thinking Globally. 12 years at Ford just could not pound that into me. They tried. And I do believe Ford was way worse at changing a part from year to year.

But on the 67-72 year trucks, maybe more, they swapped the Front/Rear brakes from Front/Rear reservoir to Rear/Front reservoir. That does not make a lot of sense. All levels had the same clearance for the MC, maybe not the booster on big blocks. I just find it very odd the swap which reservoir goes where. Especially on drum/drum brakes.

[Jonboy]

I went ahead and ordered a replacement combination valve from Summit, made by CPP. There is pretty much no info about on CPP's website, but it had good reviews. From what I can tell on inline tube's website, it looks like the trucks used the various combination valves that interchange with cars, as far as port sizes and such. I originally was looking at the Wilwood replacement combination valve with an adjust ment for the rear brakes, but couldn't get it pinpointed to which one I need. I really didn't want to start experimenting with the brakes just yet.

Thinking about it, it is entirely possible I switched the lines around when I went to the 1/2 ton master cylinder. Since I already have a new 3/4 ton master sitting here, I might as well put it back like it is supposed to be. My driveway is at a decently steep angle, and I absolutely could not get the brakes to gravity bleed. There is no fluid whatsoever going to the rear brakes, and I could not get the proportioning valve to reset. I may go ahead and swap master cylinders first and see if it makes a difference.