Rear disc brake swap, proportion valve ?

[tonyota]

Just swaped over to rear disc brakes on my buddys 40. (We did a front drum to disc w/mini truck master in spring). We installed a proportioning valve in the rear line for the rear disc, how do we adjust it? It just says increase/decrease with arrows on the valve knob. It turns several turns from one stop to the other, is there a trick, or just drive, adjust, drive, adjust....Thanks.

 

[cody c]

is this a question for a 40 series? i think you would try locking up the brakes and see what locks up first.

 

[ntsqd]

Turn it to full increase. Load it the way it is normally loaded. Panic stop it repeatedly and adjust towards "decrease" a little at a time (say ~1/4 turn) until the rears will only just barely lock up. Test it totally unloaded, adjust a little more if needed.

IME tall, short wheel base rigs seem to need less proportioning than lower and/or longer rigs.

 

[Tinker]

I heard something surprising from an authoritative source a couple weeks ago - a prop valve only reduces the spike in pressure when the pedal is 1st pushed. After that the pressure equalizes throughout the system.

 

[ntsqd]

Hum, I'd say that based on my education in these devices while working in wilwood's R&D dept. that the statement is either wrong or misunderstood.

Prop valves are pressure regulators, though they work differently than do those on welding bottles. P-valves regulate rear line pressure to a percentage (usually 57%) of the M/C output pressure once the output pressure is above a "knee point." What you're adjusting in those p-valves that are adjustable is the location of that knee point. Below it pressure out equals pressure in. Above it pressure out equals 57% of pressure in plus the knee point pressure. Kelsey-Hayes did the original development of the p-valves that most everyone uses in the aftermarket. Regardless of brand name on the unit, very few of the adjustable p-valves available in the US are not made by wilwood. I think US Brakes makes their own. I've no idea about TSM. The Summit & Speedway parts are wilwood.

The Lock Resistant Brake System (LBS), as originally developed by Stewart Components and later bought by DPI (& curiously showing a price of $0.00), is more of a pressure spike absorber. It works by slowing down the increase in rear line pressure, but does not limit the maximum.

 

[tonyota]

is this a question for a 40 series? i think you would try locking up the brakes and see what locks up first.

He drives a 40 series, not me. I'm just tryin to get the darn thing to stop. Thanks for the info.

 

[tonyota]

Turn it to full increase. Load it the way it is normally loaded. Panic stop it repeatedly and adjust towards "decrease" a little at a time (say ~1/4 turn) until the rears will only just barely lock up. Test it totally unloaded, adjust a little more if needed.

IME tall, short wheel base rigs seem to need less proportioning than lower and/or longer rigs.

Thanks for the input. I had no idea where to start with the adjusting...didn't want to hit the road & go fubar with it.

 

[cruiserunc]

I just wanted to echo a couple of things said here . . . I did this mod on a 40 I had, only using Eldorado calipers (hand brake possibilities for each caliper if you want to add it) and getting the proportioning valve adjusted correctly is a big deal. NTSQD was right on target with how to make the adjustments and you may need to tweak it later but get it right now. He clearly knows more about it than I do but having made the conversion and drove the vehicle before making the adjustment and after; the difference is significant. The before picture is; several times of nearly plowing a stop light, and the after picture is; you will have more brake than you ever felt in a 40. My 60 has good braking now but if I ever felt I needed more brake, I would not hesitate to add this mod to my 60.

 

[tonyota]

Thanks for all the input. We got it on the road today & I think it is finally adjusted. We will keep an eye on it. I also have this swap planned for my 60, now I'll know what to do.