1972 C20 master cylinder

[KLASS6]

Hi everyone,
I bought a disc/drum power brake booster from POL and the prop valve bleeding tool.
Can someone confirm that on a 71/72 3/4 ton trucks the front reservoir of the master cylinder is for the rear brakes and the back reservoir is for the front brakes?
My mechanic do not works often on US car and he is not sure if has to plug the pipes this way.
He has plugeg front reservoir/front brakes and rear reservoir/rear brakes and he has trouble bleeding rear brake even with the bleeding tool for the proportioning valve...only few drops of brake fluid are coming through the rear brakes.

[jocko]

Correct. See pic for ref - rear MC port is for front circuit (note 3/16" line) and front MC port is for rear circuit (note 1/4" line). Pic also shows proper routing thru the distribution block - note how the lines cross over. Pic is a 71, but 71 and 72 are identical.

[KLASS6]

Thank you jocko for the pic.
So my mechanic plugged the lines this way and had the valve centered with the bleed tool.
Only few drops of brake fluid came through the rear brake and hard pedal.
He then checked the prop valve rear brake outlet and only few drops are coming through the outlet.
Front brake bleeding is ok.
Do I miss something in the process or the new prop valve has an issue at the rear outlet?

[jocko]

I would check the rear brake outlet of the mc (ie disconnect where it enters the distribution block) and check for flow when you depress the brake pedal (unless you’re saying that is what you already did - I wasn’t sure if “at the rear brake” meant at the rear wheel or at the mc rear brake system outlet port). If no flow, it’s an mc issue. If good flow, I suspect the distrib block shuttle valve is not centered or the block itself is somehow clogged/faulty.

[KLASS6]

Yes I meant at the rear wheel.
Not sure if he checked the MC rear brake outlet, will ask him to check.
All parts are new, brake booster, master cylinder and proportioning valve from POL.
I assume the valve was centered as he used the bleeding tool.
I sent POL technical support a mail.

[jocko]

Not completely sure what you mean by the bleeding tool. There is a special tool for recentering the valve, but it's unrelated to bleeding the system - if you mean he beld the mc or pressure bled the system, that won't recenter the valve - good idea that you asked POL, they will know how to do it for their part. The other
less technical" method to center the valve (assuming you confirm you have good flow from the master), is to STOMP on the brake pedal real hard. It sounds silly, but it is a method of recentering the shuttle valve. Considering it's an aftermarket prop valve - it could absolutely be faulty - or it's also possible it's adjusted all the way to bias to the front (unlikely though).

[KLASS6]

I thought it needed the bleeding tool screwed where the brake light switch sits to keep the valve centered while bleeding the system at the wheel...am I wrong?

[jocko]

Ok, I see what you mean, just never heard it called a bleeding tool. Yes, I think we are talking apples and apples
I always think of the tool for recentering the valve when tripped, not necessarily for holding it centered - but now I understand what you're saying - sorry about that.

[KLASS6]

Here is a quick update.
The brand new proportioning valve was faulty at the rear brake outlet so I was shipped a new one from POL that works perfectly...problem solved.