The Stovebolt Site Forums The Parking Lot Tons 'o Fun money saving brake question

The question is, do i really need to use residual pressure valves on my 59 ton? I measured where the lines come into the wheel cylinders, 19"on the back, 18" on the front, and they are 24 1/2" where they come out of the master cylinder under the cab. i've beeen told that i need to use 10lb pressure valves if the master is under the cab, but is it really needed if the the master is higher?? if only by 5"?? It would save me the cost of 2 valves and all the fitting to make them work on the 1/4" lines that i'm using.

It depends on the master cyl. The original MC has residual pressure built in & most replacement or aftermarket MCs designed for drum brakes do also.

I got a knock-off corvette style master cylinder that came in a power brake conversion kit. i don't know if it has the pressure valve built into it. It looks just like the stock manual corvette master that i had got from the flaps. The only diffference is that the kit master has a spacer that fills up the space inside the master were the manual brake rod would go. It works just right with the booster that comes in the kit. I can only guess that it has it. How can you tell?

Hy panman, as far as I know all dual circuit master cylinders for Corvette applications are for disc/disc applications, that would mean no residual pressure valves in the master cylinder. As has been stated by Wrenchbender Ret. the original master cylinder had a residual pressure valve built into it, and most if not all master cylinders for drum brake applications have them built in. For your own safety and that of those around you on the road, I repectfully sugest that going cheap on brake components is not a wise idea.

Go ahead and buy the valves if your MC doesn't have them, which to me sounds like it doesn't. You could disassemble it and see what's in there.

I've used them on a couple trucks with the MC under the floor even though as you point out technically the MC is slightly higher than the calipers/wheel cylinders.
On a drum brake truck there is no downside to having them that I'm aware of, and as 3B pointed out drum brake systems most often have them to start with.

Grigg

If your MC doesn't have the residual valves already, I would highly recommend the external ones. It may cost a little now (not all that much really), but you'll have more even braking, less frequent shoe adjustment, and more a responsive brake pedal. Having the MC mounted up higher probably helps a little, but you're not going to produce 10lbs of pressure that way.

I kinda figured that i should get them but i was just wondering if they were really needed if the master was even just alittle bit higher under the cab. With no e-brake and an auto tranny i can't take any chances with stopping

...just my experience. Years ago I didn't even know what a residual valve was!
I put my M/C under the floor of my 37 and I'm running disc front and drum back. I've had this truck on the road for about 11 years now and no problems what so ever.

Now,2011, I have a panel with "currently" drum front and back.
I do have the master cylinder and the booster under the floor on this panel same as the 37. I like a clean firewall.
Anticipating disc on front somewhere in the future I did decide to put a 10lb residual valve on the rear. When I ordered I actually went ahead and got two of them not knowing then exactly how I'd do things.
Well, the first valve I put on I could not get to stop leaking around the threads. So, I changed it and put the other one on. Still could not get it to stop leaking although it was only a drip or two a day. Yes, I even changed the line and I tried tighten, loosen, tighten, used weatherhead/eaton fittngs and I know I had the right threaded fittings. In the end I just took it off and put a union in the line. It'd help if we could get a residual valve that the brake lines would screw straight into on both sides without having to use a "reducer". So, "my" experience hasn't been good....but the panel stops just fine. If it was to be a hot rod I'd resolve the valve leaking problem... and probably will on a cold winter shop day.

I do like the idea of having that 10 lb valve to help overcome the rear spring tension but is it really necessary. You be the judge.
...as for brake adjustments you can put automatic adjusters on your truck now sweat.

Hy Guys, an old GM engineer who's name escapes me at the moment said "parts left out cost nothing, and cause no service problems, so you can be sure if we put it there it's there to do a job". All drum brake systems I've ever worked on have had residual pressure valves, they keep the cups against the wheel cylinder bores to help prevent air/moisture from being drawn into the system when you release the brake pedal, and they keep the system "charged" so your first application actually applies the brakes, rather than repriming the system. I'm not an engineer, but I do repect the work they do, and I'm not about to try and out think them.

Would a 1 1/8" master cylinder be better than a 1" with a drum/drum system or a disc drum system in these applications?

If it has power the 1 1/8 would be better. Without power I would use a 1 in.

I'm using a 1 1/8 master, on the power brake conversion kit, it was a pain in the butt to find a kit that has one. The orginal manual master was 1 1/8 with all 1/4 inch lines. what really sucks is that the pressure valves are all 1/8" now i have to buy the adpaters to make it all work