The Stovebolt Site Forums The Parking Lot Big Bolts '52 gmc 450 COE brake sizing question.

Well, I've finally got my "mad money" stash built up again. I've got my COE on the road. When I bought the truck the hydrovac had been bypassed I just assumed it is toast. I rebuilt or replaced all the wheel cylinders, and brake hoses. After I adjusted the brakes the truck will slide the two front wheels. This only takes moderate pedal pressure when the truck is empty, but when I put an empty dual wheel, tandem axle trailer behind the truck it takes considerable pedal pressure to lock up the wheels. Before putting on anymore load than the empty trailer I want to get some type of power assist. While I’m at it I’d really like to convert over to a dual chamber system. I plan on working this truck when I’m done. The GVW is 19500lbs. I want the truck to be safe up to this weight. The options I’m considering are:

1. Rebuild the existing Hydrovac unit. This would probably be the easiest and perhaps the cheapest, but it doesn’t accomplish the dual system goal.

2. Put in a vacuum assist system from a newer gas powered big GM truck. There is room under the cab floor for the 10 ¾” booster behind the engine. I would have to fab up some mounting brackets and re-route the pedal rod. My uncle own a machine shop so this shouldn’t be too hard to solve.

3. Put in a power steering assist system (hydro boost?) from a newer diesel powered GM truck. This could be easier than the vacuum assist because it is a smaller set up than the vacuum boost canister. I would have to add a power steering pump to the 302. This may not be so easy on COE without pulling the front end sheet metal.

My questions for the masses are:
1. Is there any merit in one type over the others? Safety in the #1 goal.
2. How do you size a donor master cylinder? All I know is that the service manual says that my stock master is a 1 ½” bore. I’ve been looking at a master for a 1997 5500 Chevy. (Alaskabob used this setup on his truck with a through the firewall pedal) There are two part #’s listed, one with either a 1 ¾” or 2” bore. I don’t know if either of these are appropriate or not. These masters appears to be used with either a vacuum or hydro boost.
3. If I decide to go with a hydro boost system where could I find power steering brackets and a pump for the 302 GMC? I am not planning on adding power steering to the truck at this time, but might later. Will a power steering pump live supplying only a hydro boost?

I know this a long post, thanks for sticking in there. Any advice is greatly appreciated.
Scott

I have a 56 6100 that I have been building for heavy use as you are, so braking and safety as well as availability of replacement parts are most important. So I contacted the guys at http://www.abspowerbrake.com they were very friendly and knowledgeable. I let them know my gross vehicle weight the size and type (drum/drum) of brakes and the bore size and number of wheel cylinders at each end. Armed with this info they set me up with a bolt in frame mount that uses the original pedal a hydro-boost unit and appropriate master cylinder that they claim will provide superior braking power and safety over the original system.
As I have made the decision to upgrade to power steering at the same time I then contacted the folks at http://www.agrperformance.com as I decided on using their Rock Ram steering assist with a power box from a scout two. I let then know the specs on the hydro-boost unit and the set me up with their rock ram, a high output pump built specifically for the system as a whole and hoses and fittings that should cover all my plumbing needs.
Grigg on the other hand has vast fabrication skills he built the same system minus the rock ram for his 48. He built his own bracket and acquired the parts needed from wreaking yards for medium and heavy duty trucks. I am sure Grigg did it much cheaper than I but not sure how much I spent about $1200.00 on the total brakes and steering.

HeavyHauler, thanks for the info, I'll give those folks a call the first of the week. I too am concerned with building a safe system, in today's lawsuit soceity you've got to be. Thanks again,
Scott

If you rebuild every thing you have and put in a hydrovac, you will have plenty of stopping power. I have several 50's models with hydrovacs and they have very good stopping power. I drove these type of vehicles when they were new. If you absolutely want a dual system, you need to get a system that is matched. Either take an entire system from a later vehicle, or get a complete aftermarket system that is engineered to work together. Don't try to mix and match or you will have either front or rear wheels grabbing which is very dangerous on wet pavement. The only big improvement I have seen in brakes on larger trucks in the last 50 years is ABS. That goes for air brakes also.

Crenwelge,
Thanks for the advice. I have indeed rebuilt or replaced everything except the hydrovac and a few of the steel lines on the truck. Your experience with these trucks is valuable. I don't really have a prefrence which system I end up using, but I was under the impression that a dual chamber system would be the safer way to go, so that a loss of pressure on one end won't lead to a complete brake failure. It's not that I don't trust the hydrovac either as we have 4 other trucks here on the farm set up that way, the newest being a '78 c-70. I just thought that if I had to replace/rebuild the hydrovac now, for a little more money and effort an upgrade might be worth it. But like you say a cobbled together system could be a lot more dangerous than a properly maintained stock system. Decisions, decisions...