Under-Cab Power Brakes and Dual Master

[Greg Coffin]

Hydroboost Feedback

Recently I received some feedback from a fellow that used a bracket to mount a late-model Dodge hydro booster and master cylinder onto his Power Wagon. I'm posting this so that others may avoid similar issues with their hydro boost systems.

The first issue was the plastic reservoir on the master cylinder was tall enough that it hit the floorboard of the cab. He ended up relocating the reservoir to the firewall and plumbing feed lines to the master cylinder. There are also aftermarket master cylinders that have remote reservoirs to mount on the firewall.

The second issue was that the brakes were very touchy at first. There was very little feedback on the pedal, and he could lock up the brakes with only 2-3" of pedal travel. After some debugging he found that the oil cooler that was installed on the return line from the booster was creating excessive back-pressure in the system, which caused the booster to malfunction. Once he removed the oil cooler, the system operated normally.

[turbobuick1]

Interested in buying a kit from you

I sent you an email a day or two ago; I'm looking to purchase a kit from you. I'm interested in as much brake assist as I can get, without going to hydro boost, so maybe a 7" or 8" dual diaphragm booster.

Also, how is that brake cylinder bore size working out for you?

Please contact me when you can! Thank you!

[Greg Coffin]

Just finished up a new batch of brake brackets. One is spoken for, the rest are available. $400 each including shipping. Email me if you are interested - just click on my name at the top left of this post.

[Greg Coffin]

Floor Access

Here's a photo of the master cylinder location in relation to the floor opening. The front chamber is easy to reach and fill, the rear chamber will require a small funnel to fill. Both bails are easy to reach from inside the cab.

I'm using a 7" single diaphragm booster on my system. A 7" or 8" dual diaphragm booster will push the master cylinder forward another 3"-4", which is just about perfect for access.

[Kaegi]

pivot question.

Looking great Greg. what is the distance on the stock brake pedal mounting pivot to hole where rod attaches? I think the bell cranks on your bracket need to be that length to retain stock pedal ratio.

[Greg Coffin]

I thought the same thing

I think that would be true if the bell cranks were parallel, but they're not, and they rotate in opposite directions.

When I built my first bracket, I made the lever arms the same length as the stock pedal arm, which is 2-1/2". But what I found was that I was only getting about 75% of the total capacity of the master cylinder when the brake pedal hit the floor. I changed the ratio of the lever arms so that now I'm getting 90% of the master capacity at full pedal travel.

[Kaegi]

length

either way you are quite close to the same length as pedal. less ratio might even help pedal feel.

[vcpowerwagon]

Hi Greg. Just a quick update on my installation of your bracket. Instructions are pretty straight forward. I am installing this one on a 1950 B1PW. There are few custom variations but that is to be expected. I am at the point where I need to come up with a mount for the brake pedal return spring. As you know there is no Clutch Slave Cylinder bracket on a B1PW. Looks like that is what you used to mount the return spring on the WC63 frame. You also state in the instructions not the use the brake rod itself or any of the pivot arms.. I love a challenge... I will take a few pictures of what I come up with. So far so good. Thanks Greg.
Martin

[Greg Coffin]

Thoughts on Return Springs

You are right, I fabricated the clutch master bracket, which helped in finding a place to mount the return spring anchor point.

There were a couple reasons I recommended not attaching the return spring to the brake rod or lever arms. However, I'm not a certified(able?) engineer, and I'm open to input on these points. (Actually, I'm reconsidering my position as I write these ideas.)

First, I was concerned that any spring load on the rod or lever arm might make it harder to adjust the free play between the booster and the master cylinder. But thinking about it now, a spring on the lever arm would keep the rod and pedal constantly in tension, which would keep any slack from creeping into the adjustment. That might be a good thing.

Second, I was concerned that attaching the spring to the rod or lever arms might allow the brake pedal to vibrate at certain RPMs or road speeds. However, putting a return spring on the lever arm would remove any slack in the rod, which would keep IT from vibrating as well.

I was also concerned that whatever spring I chose to use might not work for every application. However, I could use an adjustable tension anchor point, so the tension could be adjusted. I like that.

I'd REALLY like to hear feedback on these points. The return spring issue has bothered me since I designed the bracket, and I'd like to find a good solution for it. If there are good reasons to incorporate the return spring into the bracket, I will build it into the next batch of brackets. I could also offer a retrofit kit to people who have earlier brackets.

Thanks much!
Greg

[Greg Coffin]

Doh!

I thought I had figured out the PERFECT way to install a return spring on the bracket - simple, elegant, and easy to adjust.

And it pulled the wrong way.

Back to the drawing board.
-Wile E Coyote

[vcpowerwagon]

M/C update

Hi Greg. Just an update on my install. I drilled and taped my block for the dual circuit system out of aluminum. I tack welded a nut to the inside frame and ran a bolt through the block with star washers to keep it from rotating.
Next pic is of my method of attaching the brake return spring. I built up some welding material underneath the tip of the brake rod being careful of interference. Drilled a hole the diameter of the original spring extension wire and modified its length. I can still use the original brake spring and its other attachment point on the frame.
Last pic is the new system plumbed up. I still need to bleed everything yet but I am every happy so far with your design.
More pic's to follow.

Martin

[Greg Coffin]

Martin, I like how you attached the return spring - very elegant! The hole in my support casting didn't seem big enough to fit the rod for the return spring, but I will have to look at it again.

Nice work on the junction block for the hoses. I've read different things about how to plumb the master cylinder to the front and rear brakes. I ended up switching my lines so that the front chamber goes to the rear brakes. Apparently most master cylinders are designed so that the front chamber activates first (a bit counter intuitive) so that the rear brakes energize just before the fronts, to help keep the vehicle running straight down the road. I don't know if it is actually true, or how much difference it makes, but all of the medium size trucks I looked at were plumbed this way.

[vcpowerwagon]

Front and back curcuits

Thanks Greg for the heads up with plumbing of the brake circuits. I will do some research and decide which way to go. I have yet to bleed the lines...

Thanks again
Martin