dual circuit MC on WM300

[Kevin Mienke]

I am currently doing the Helitool disc brake swap on the front axle of my 1961 WM300. I am finding this “cookbook” swap to be very easy but I do have some questions regarding a possible upgrade in the master cylinder setup.

As long as I am upgrading my brakes, I think it would be wise to upgrade to a dual circuit master cylinder as well, separating the front discs from the rear drums. This is something the cookbook doesn’t cover on the WM300.

Understand that I know only enough about brakes to be ignorant-plus.

Would the factory dual circuit MC used on the ’67-’68 PW be a bolt-up in the OEM location on the ’61? If so, what modifications regarding residual pressure and proportioning would be necessary to use this drum/drum MC with a disc/drum setup?

[mannyc]

late ffpw upgrade

kevin - if you can find a late pedal assembly, you can bolt it in place of your current setup. i did it in my command car - a wc 3/4 and the hardest part was finding the pedal assembly. i upgraded the master to the one used on the helitool m37 dual master conversion. i am running unboosted and have had no problems whatsoever. 4 wheel discs. the swap allows you to reuse your stock floor board but one with an access can be had from midwest military. once completed, it looks completely stock to the causal viewer. it is a great uprgrade.

[Kevin Mienke]

Thank you Manny but I am still unclear on this thing.

The master cylinder Helitool used on the M37 is from a 1980 Chevy C30 1 ton truck. It is a disc brake unit that does not have residual pressure check valves built into it. My understanding, from reading his web page is that this unit, because of the 1-5/16” bore size and the fact that it does not have check valves is perfect for a disc/disc setup. He also indicates that it is good for the disc/drum setup I am using but he goes on to say something that I get hung up on.

Here is a quote from his page, “If you have disc brakes on the front only, you probably do not need a proportioning valve since the rear drums are so inefficient that the front discs are already providing 70% of the stopping power.”

Am I being naïve, or do I want my rear drums to work efficiently in combination with the front discs? It seems that I will want a master cylinder that is specific to a disc/drum setup with a residual pressure valve for the rear brakes only…or… a disc brake MC like the one he uses with an in-line residual pressure valve for the rear??????????

This is all clear as mud to me…..I much prefer bodywork!

[W_A_Watson_II]

Kevin,

Have you figured out how to mount the 1 ton Government Motors MC yet? I gave up and will by buying one of Ray's hanging pedal kits.

[Kevin Mienke]

Kevin,

Have you figured out how to mount the 1 ton Government Motors MC yet? I gave up and will by buying one of Ray's hanging pedal kits.

Hi Will, No I haven't done anything yet except try to research what my best options are. At a glance, Ray's MC looked like it might only require the fabrication of a two bolt bracket, but these things are never as simple as they seem. If you've tried it and given up, I won't waste my time with it. Thank you for your input.

[mannyc]

dual master observations

if you find and install a 1967 on flat fender brake pedal assembly, you can relocate the master cylinder under the floor. once installed, you can without modification, install the helitool suggested master cylinder. i installed the master using discs front and drums rear at first. master cylinder performed much better than stock setup with drums. because i liked the discs and my command car is a driver, I installed discs in the rear too. same master. i have great pedal and have not encountered a situation where i feel any other changes are necessary. if you run drums in the rear, you might want to add a check valve in the rear circuit to maintain some pressure in the rear lines so that the shoes dont fully retract. i think a non boosted setup works great and i felt no need to complicate things. i ran an identical system in my m37 with a little red express truck motor. truck had tons of power and highway gears with 1100 16 radials. no need for boosters there either.

finding the assembly is the hardest part. i can take a pic or 2 if that would help.

manny

[Kevin Mienke]

Thanks Manny,

My current pedal assembly is the later 251/420. My understanding is that this pedal assembly didn't change after 1961. So I should be ok there (I think).

Obviously I have two conflicting reports on whether this will work under the cab of a civilian PW though. Possibly there are differences in the MC mounting on your command car and the mounting on a civilian PW?

On the PW there is about 3" clearance between the frame and the brake pedal. The original MC attaches with a three bolt flange to a bracket in this tight area. Anything that is going to easily adapt to the existing bracket would have to be 3" wide or less at the flange.

[NNICKB]

Kevin,

My '67 WM300 has a dual master cylinder with what appears to be an aftermarket brake booster. The truck stops very nicely in around town driving, with just a light tap on the brakes. It was only after I had to make a panic stop that I realized there was not much more available! The point is, despite the simplicity of a brake system, there is a lot of engineering that goes into making it work properly...the use of the "cookbook" may not be the best approach.

Drum brakes are, to an extent "self actuating," meaning that when you press the pedal and the shoes come in contact with the drums, the turning drum actually draws the shoes further outward and compounds the braking force. A disc brake operates on a completely different principal, that is, the pads clamp onto the rotor by solely by the force of whatever outside pressure is applied through the lines. Bottom line is that a master cylinder designed for drum brakes will not have sufficient line pressure to operate your discs. Indeed, with a disc brake setup, a larger capacity m/c with some sort of power brake booster is virtually mandatory.

I am a little skeptical about the suggestion that your back brakes are not important because most of the stopping power comes from the front. There is a certain amount of truth to that...when you are dealing with a passenger car or an unladen truck. HOWEVER, a fully loaded medium duty truck derives some 80% of its stopping power from the REAR brakes. This means that if you intend to load your PW to anywhere near what it is capable of hauling, your rear brakes become much more critical.

Remember also that most PWs came with 5.83 gears and rarely saw speeds over 50 mph. Lots of people are going with big tires and/or 4.89s these days, which helps the road speed some. I am told that the 5.83s were recommended for steep terrain, which I had assumed was to help the truck climb hills. However, I can tell you from experience that the low gearing also helps control the truck going DOWNHILL with a heavy load, and that you will quickly fade the brakes if you rely on them too much. Point is, the brakes on these trucks are pretty marginal (but then, you knew that.)

If you are running your truck in heavy traffic at prevailing speeds, and you are carrying heavy loads, you probably want your rear brakes to be up to the task, along with the fronts. I have seen a lot of these trucks outfitted with small block Chevys, with the stock axles and brakes. Hey, these motors fit nice, rev fast enough to work through the stock gears, and don't have enough torque to break stuff. But if you see one flying down the interstate, better get out of the way!

Many companies are making disc brake conversions for older cars these days, and the best ones, IMO, come with a fully engineered combination of four wheel discs and matching power boosted dual reservoir master cylinder. They also typically have an adjustable proportioning valve so you can fine tune the braking to work best for the size and weight distribution of your particular vehicle. As for your swap, I'd prefer to see you using the front brakes, rear brakes, and master cylinder all from the same donor, and obviously a truck of similar size and capacity to yours. The C30 would be a good choice, I assume, though I don't know what will be needed to adapt everything to your truck. At this point, I will confess I have never done one of these swaps, so should defer to someone who has.

Only other comment I would make is that whatever you come up with, make sure you take it out and thoroughly test it under different speed and load conditions so you have some idea of what you have created!

[mannyc]

nick b - some good points

i believe the helitool alternative to be superior to the stock system. gaining 2 channel brakes is a major safety improvement even if that was the only change made. adding front and possibly rear discs improves the stopping power over stock - you are correct that the proper master should be used. a master that has internal check valves for the drum rear and free flow for the front discs is a popular gm offering and should be employed. if a boost is desired, proportioning valves, check valves, etc. should be used and dialed in. your late ffpw with a dual master came with an booster system from the factory. you should research what was there and investigate what is needed to bring it back to stock performance.

i have a stock 230 in my command car which is much lighter than a ffpw. dual discs with a dual disk master (unboosted) works well. when needed all four wheels will grab and hold. i have power steering and if i ever felt the need for a boost, would go with ps powered variety.

[NNICKB]

Power steering??? Now there's something I'd really like to have...

[Kevin Mienke]

Since the last posting, I have had another conversation with both Manny and Will and things are now clear to me. The 1967-68 PW had the dual circuit MC mounted on the back side of the bracket. All previous models had the MC mounted on the front side of the bracket. Mounting on the back side allows for the use of virtually any master cylinder, the lines are protected and there is no issue with tight area, steering box, etc.. The 1967 pedal assembly is different in that it is configured to actuate the backward mounted MC. I will be looking to find and purchase one of these pedal assemblies, just as Manny had suggested. If I'm patient, I'm sure one can be found.
As always, I appreciate the insight offered by the good folks on this forum.

[Jason Mills]

Since the last posting, I have had another conversation with both Manny and Will and things are now clear to me. The 1967-68 PW had the dual circuit MC mounted on the back side of the bracket. All previous models had the MC mounted on the front side of the bracket. Mounting on the back side allows for the use of virtually any master cylinder, the lines are protected and there is no issue with tight area, steering box, etc.. The 1967 pedal assembly is different in that it is configured to actuate the backward mounted MC. I will be looking to find and purchase one of these pedal assemblies, just as Manny had suggested. If I'm patient, I'm sure one can be found.
As always, I appreciate the insight offered by the good folks on this forum.

I have been wondering, (since reading about Manny's suggestion) if a 1967-68 style pedal assembly could be made.. it would seem to be a great upgrade. What do others say about such an idea? I made a bracket earlier this summer to adapt a late model ford master cylinder to my WDX, and although it does work, I cannot say I am delighted with the set-up. Maybe an attempt at copying what Dodge did would be a better solution.