[george kujanski]

Iaddedmanual front disks to my '67 GTO, also Using a combination valve.

On hard braking, the rears lock up, fronts don't. Should I add an adjustable prop valve at the MC rear reservoir tube?
I suggest the location because of ease of adjustment.

George

[JLMounce]

It actually sounds like you may have a malfunctioning, or improper combination valve. Did you receive a new combination valve with your front disc brake kit? If not, did you re-use the factory drum/drum distribution block?

Here's a link with great information on how a combination valve works. https://mpbrakes.com/combination-val...nation-valves/

Unless you're fine tuning for performance driving, a proportioning valve is almost never needed over a proper combination valve. What it sounds like is happening in your situation is that the hold-off valve is either not part of your combination valve, or isn't functioning. You may also have a displaced shuttle in the combination valve that is partially blocking fluid to the front brakes. I am concerned that you're not able to lock the fronts. you're looking at the rear brakes as having the issue, but I think the fact that you're unable to lock up the front wheels is the more concerning symptom. Fixing that will likely have an effect on the rear.

In your case you want to make sure you have a disc/drum combination valve. Not a distribution block designed for drum/drum applications, or a combination valve designed for disc/disc applications. You need the hold off valve to keep the disc brakes from applying before the drums and causing the nose to dive. That rapidly unloads the rear tires and will cause them to lock prematurely.

If you bled the brakes without a shuttle centering tool installed in the combination valves, it's possibly you tripped the valve. If that's the case you need to remove the valve, recenter the internal shuttle, then rebleed the system with a shuttle center tool installed.

Verify all this first and make sure the system is properly bled.

[george kujanski]

i'm sure it's properly bled. The combo valve I bought is from Inline tube advertised for the aftermarket front disc upgrade with the hold-off and prop valve built in. The internal shuttle should be fine; i used the tool when bleeding and I have the electrical connection hooked up, showing no brake fault.

Perhaps I'm wrong saying that the fronts don't lock up.......... I tend to get off the brakes when the rears lock up and I get the annoying tire squeal so i probably have not REALLY tested them. My expectation is that they all should lock up at approx the same time.

george

[JLMounce]

Under a panic stop, with a properly functioning combination valve, you should have the front and rear locking up with each other, or with the rear lockup occurring shortly after front lockup. This is to prevent the ass-end of the car from wanting to swap places with the front.

When was the last time your drum brakes had an adjustment done on them? If the shoes are very far off the drum friction surface, the hold-off valve might still be applying the front brakes first and unloading the rear.

[Skip Fix]

Lots of cars lock the rears up as weight shifts to the front and if you do not have stiffer front springs the rear unloads so loss of traction. Why back in the day Guldstrands stiffer springs vs Herb Adams soft springs bigger sway bars always worked better for me. I'd go too hot into a turn then brake Back in the day GM even had one for Corvettes as a GM part number. Different sized tires fvcvbg from stock, different shock valving and spring rates all undo what the factory engineers designed. And they usually did not design for "hard driving".
My 78 TA when new did that until I changed springs and added an adjustable Vette valve , then when it started leaking used a Mopar Direct Connection valve.
I have always plumbed then in after the combination valve.

Now when my folks had a ' 70 Vega wagon with the hand E brake we would be driving down the road and pull it screeching tires while going straight We even shoved a 409 short block from a wrecking yard in the back and drove it home for my Impala!

[HWYSTR455]

Bad shocks can make them lockup too.

I use a combo between stiffer springs and bigger bars, not sure the Adams approach is clear on the whole topic. Going to an aftermarket, free-moving control arm bushing setup, you need stiffer springs to make up the difference. Something many never took into consideration I think. (Especially in comparison tests).

I used the Direct Connection valve for years on the rear line, no combo valve. In more aggressive driving, you would have to give the thing a turn. I had mine mounted on the e-brake pedal bracket, could adjust while driving.

One thing to note too is that some masters have the front/rear ports reversed. Just check to make sure on the one you're using. Usually, but not always, the rear brake reservoir is smaller in size. Some are same size front & rear.

Here's a basic overview type tech doc at Pirate Jack's:

https://piratejack.net/content/PJTG.pdf

"Most GM master cylinders use 9/16-18 threads for the front ports and 1/2-20 threads for the rear ports."

The Pirate Jack doc is actually an MBM doc, can go to their' site too for additional info.

https://mbmbrakes.com/tech/

MC info:

https://mbmbrakes.com/which-master-cylinder-is-for-me/

.
.