No proportioning valve

[72LB]

Wrapping up the brakes on my 72 I have found it doesn't have a prop valve. I thought it might have been on the frame but was wrong. When I bought it and limped it home it stopped ok with the exception of one of the rear wheel cylinders leaking. Funny thing is unless one of the po's replaced the lines years ago it doesn't look like it had one. I'm assuming it will be ok but should I install one?

[morepower71]

I wouldn't worry about it unless you start driving it and find the rears lock up way before the front. Trucks seem to need them more than cars but depends on the master cyl and if you have done any changes to the system.

[davepl]

It's a "combo" valve in our case because it does distribution AND proportioning. Are you saying your lines come right out of the master and go to the wheels?

It should look something like this:

[Keith Seymore]

I have the '72 pickup brake chart here in front of me.

All models have a combo valve except P30 motorhome and C30 w/H22.

They all use the same one.

The third picture defines the combo valves characteristics in terms of meter (pressure), split point (pressure), and slope after the split.

K

[72LB]

Originally Posted by davepl

Are you saying your lines come right out of the master and go to the wheels?

Yes, front to front back to back.

[mattfranklin]

Redoing my '70 with a stock '72 front disc setup. Yes, it should have one. The original combo-valve would be iron as shown in davepl's gorgeous engine bay pic, but the new replacements are aluminum as far as I've seen.

[72LB]

Hm ok something else to add to the ever growing to do list for this one.

[davepl]

Originally Posted by 72LB

Yes, front to front back to back.

That could be an issue as well - I'd have to double check, but I think it's actually reversed from that - the rear of the master does the front, the front of the master does the rear wheels, at least on the stock setup.

Sounds like you need to grab the 1972 combo valve and plumb it in, ideally like the picture I posted above.

I can't say that's the ONLY way it'll work, but I know it DOES all work nicely that way.

Oh yeah, and don't forget to paint the valve with some kind of brake-fluid-resistant paint if you can, mine's already flash rusted...

[Zeroman]

I have been told by "learned" people the position of the brake lines is determined by the master cylinder.

Front disc brakes typically require the larger of the two reservoirs due the to volume of fluid required by the caliper pistons. That would be on disc/drum system.

That is not to say it will not work the opposite way, but that is the best situation.

Disc/disc systems sometimes require an adjustable "proportioning" valve to properly adjust front to back brake pressure.

[72LB]

The master cylinder on this one has equal reservoirs.

I'm going next week to look at a parts truck maybe I'll get lucky with what I'll need for this one.