1956 GMC F370/374 LCF Brake Upgrades

[DoctorGlenn]

I'm in the process of planning the major upgrades to the rollback carrier that I just bought (see my earlier thread). The truck has a 21,000 GVWR and is equipped with the Bendix Hydrovac for braking assist. I know these can be rebuilt but lets face it, they're 60 year old technology and I plan on using the truck for hauling, so an upgrade is in order. I'm thinking Hydroboost would be a good way to go. The truck already has power steering, I could get a dual circuit system out of this, and I'm sure I could package everything under the floor in the current master cylinder location. And while I can find vendors and information for doing a swap, everything seems to be geared towards cars and pickups and so far I've come up empty handed on making the swap on a 2-ton truck. That leaves me with some questions.

First, almost all of the information I'm finding assumes an upgrade to disc brakes at least on the front. Some forums say that using Hydroboost with four wheel drums makes the braking extremely touchy. Hydratech doesn't recommend it saying it causes excessive grabbing and left to right front brake pull. But over on BinderPlanet all seems well with the setup. Does anyone here have any experience upgrading a truck like mine? Or is there some option out there to upgrade my front brakes to discs from a later model medium duty truck? That's actually something I'd be interested in to get the better braking and if that is possible, would I be able to keep the 6 on 8.75 bolt pattern wheels I have now?

Again, thanks for any thoughts or information you might have.

- Glenn

[sweetk30]

I called a few hydroboost swap places not to long ago and they all basicly said hydroboost and drum brakes front and back is not the best setup .

at work we have a 82 c60 that needed a hydrovac unit / master under the cab . rockauto had one for under 400 bucks.

the drum brakes were adjusted and she does good .

I also tried to research the brakes on med duty stuff and the info seems to be a big black hole of nothing out there unless you know the correct person .

[DoctorGlenn]

I know what you mean about the medium duty information. I haven't even been able to find out when everything went from hydraulic to air. A major issue for me is wanting to go dual circuit which can't be done with a single hydrovac unit. I'm going to call Hydratech today and talk to someone in tech support. It seems that the larger swept area and greater weight of our trucks *might* make a difference. If not, I guess I'll just look into using a standard vacuum booster on a dual circuit master cylinder.

Thanks!

- Glenn

[sweetk30]

dont forget if you go with any booster unit on firewall then you will need a stiffener plate like the factory did or you will blow out the firewall in short order from all the flexing .

and the last problem is i could not find a stockish master that could match up the piston dia and volume output of the old . so you would be down grading in size even on the biggest one i could find over the stock stuff.

[DoctorGlenn]

Whatever route I end up going will get mounted to the frame under the cab so I can use the original pedal setup. And the engine compartment on the LCF trucks is a little cramped, especially with a BBC so I don't want to take up any of that space if I can help it.

- Glenn

[Okiefromwatonga]

I was thinking about this one my self a while back and what I came up with is either use the power steering pump and hydro booster from a TopKick on a custom bracket on the frame rail or figure out the same booster/master set up on one of the VAN 90 degree adapter on the firewall

[DoctorGlenn]

Talked to a guy at Hydratech yesterday but came away with more questions than answers. First, he blamed the problem with hydroboost and drum brakes on the air gap between the shoes and the drums. When I asked him why that was any different than the air gap between the rotors and the pads he want off on tangent about 1500psi and some other stuff I can't even remember it was so disjointed. I asked him if dropping the pressure coming out of the pump would help (Borgeson makes a kit to lower the pressure of a modern Saginaw pump to what appears to be a livable 700psi for the stock steering assist cylinder) he then claimed that applying the brakes would remove all of the assist from the steering. I'm no expert, but I thought the pump, steering gearbox and hydroboost unit were all plumbed in series so any drop in system pressure would be felt everywhere.

So I've got a couple more places to call next week where hopefully someone can give me some hard data.

- Glenn

[buggy5872]

Get a front axle from an Isuzu NPR truck and grab the rest of the brake stuff. It still has the same bolt pattern as these trucks and the width is almost the same. You have to replace the studs so you can run the correct lug nuts. The metric lug nuts are ball seat and will not work with conical seat wheels. The NPR has disc brakes. Big Bertha has the NPR setup on it. I'm going air brakes when I get to my 59 GMC F370. I have a set of axles from my 58 GMC 600 that I am going to use on it. air brakes are so much easier and cheaper to rebuild. I'll PM you my number. I'm probably a good resource for you.

[DoctorGlenn]

Okay, it's been a while and between work and winter finally deciding to arrive things are moving slowly on the truck. I picked up a wrecked 2012 NPR-HD a couple of weeks ago for what I thought was a bargain price and a semi-local truck salvage company is pulling the parts I want in exchange for the rest of the truck that I don't need.

I'm getting both front and rear axles, all of the steering and all of the brakes. For the brakes that will give me front discs, dual circuit master cylinder, and the vacuum booster. I'll also be able to replace the steering gear/slave cylinder setup on the truck now with modern power steering as well as get more reasonable rear end gearing (5.38:1 rather than the 6.61:1 high range it has currently).

I should be getting the parts in a couple weeks (the shop shut down for the end of the year) and I'll post more when that happens.