The Stovebolt Site Forums The Parking Lot Big Bolts Dual master cylinder upgrade

Any fine folks recommend over the counter options available for upgrade for brake master cylinder 1948 Chevrolet/Maple Leaf 6400 ? Has hydro vac system which was rebuilt while back but can’t get to function properly, brakes parts all fairly recent and bled numerous times pedal remains very hard to push and sometimes brake drag, rather than mess around with single line brakes looking to maybe go dual master and bypass booster and just run standard non power brakes I only use it around the farm and short trips to get supplies. Any insight much appreciated.

Highlander, Your post I think will get better response in the Big Bolts forum as that's where you will find all our big truck folks hanging out and asking such questions. Your post has been move there for the best possible help and feedback. Very nice truck by the way!

Thanks for moving post, All The Best.

Highlander, just for giggles you could hydraulically bypass the HydroVac and see how the manual system acts, it kinda' sounds like more than one problem. I ran my 2 ton for a few months with no HydroVac plumbed in, all new brake lines and hoses, rebuilt all 6 wheel cylinders and the master. When I rebuilt the HydroVac, everything came together and is working very well.

Years ago, my original Hydrovac partially failed, it wouldn't provide boost anymore. With both feet on the pedal I could almost stop it.. wound up pulling the emergency brake at the end.

Wouldn't recommend running it with no power brakes, outside the yard.

Will try pressure bleeder will have to make a filler cap fitting of some kind for master as don’t have correct adapter, I have tried vacuum bleeder and numerous manual bleeding attempts with no luck, booster was rebuilt not that long ago and vacuum coming to booster working fine also has reserve canister and all parts been replaced last 5 years or so, was just putting out feelers if there is a reasonable dual master upgrade before going through original setup and replacing parts, don’t think I’m interested in a firewall mount system as truck is pretty original, thanks for the replies

It's sort of amusing, in a potentially tragic sort of way, to listen to people who think the folks who designed hydrovac-assisted brakes could have made these big trucks stop just as well without spending all that money. I guess those guys were just determined to waste thousands of dollars of GM's money for no reason- - - - -right?
Jerry

Not sure I understand the question ? My 1969 Loadstar fire truck has hydro vac and it works just fine was more looking into a dual master option if someone has found or used a compatible unit with minimal modifications I probably was not articulating my intention correctly.

Unless you have the engineering background to consider all the variables involved in substituting non-stock parts for what was developed specifically for the vehicle, you're creating a lot of liability for yourself, and creating a potentially deadly hazard to everyone who has to share the road with you- - - - -and yourself. A modified vehicle- - - -particularly one the size of a big truck- - - - -is an ambulance chasing lawyer's wet dream. Modifications might seem like a good idea, at least until you're the guy on a witness stand explaining why you ran over that SUV full of kids, driven by a soccer mom.
Jerry

Oh I get it you’re the local smart guy.
Highlander

Highlander, some of the Task Force big trucks had dual systems with dual HydroVacs, I can look in my shop manual and see if there is more detail but I'm pretty sure that system was used on tandem axle tractors, just a thought.

One of the big Winnebago motorhomes used that dual hydrovac system with a 2-chamber master cylinder- - - -back in the mid 1970s, I believe. We had one in my shop for some brake work when I owned a diesel shop and road service, and it was a huge hassle to bleed the brakes.
Jerry

When you say dual master cyilinder, I think of the two-compartment master cyilinders that modern vehicles use to provide redundancy in case of a brake line failure.

I think for the big bolt trucks, throwing a power steering pump on and using a hydroboost MC is probably the best option for moving away from a hydrovac. In any accident you want your truck to be able to stand on its nose when you punch that brake pedal.

I have converted my "58 4400 (Thor) to a Hydroboost. I can tell you first hand this thing will stop like NOW.. Adding the power steering pump and lines was not that complicated, getting everything routed was the hardest part.

Thanks for all the replies, I came across couple of aftermarket booster and pedal units that appear to bolt right in, ones complete with master have only the 1 inch bore so looks like would get larger master separate, price is around $300 so not bad at all. I will go through factory system again see if I can get the booster system working first since it’s all been replaced about 5 years ago it’s sat in the barn most of the time and never got around to trouble shooting the issue fully I have a square body dumper that gets most of the daily use on the farm.

All The Best
Highlander

I worked on an '80s Chevy chassis school bus that was about a 60 series (similar in length, weight, WB, tire size, etc. to my 1965 60 series) and it came from the factory with a big ole' vacuum boosted dual circuit master cylinder. Looked and worked exactly like a super sized version of what's on a car or pickup truck.

So that leads me to believe that retrofitting an 80 year old design single circuit, split master/slave cylinder brake, to a modern day dual circuit setup (granted, I am broadly assuming a competent conversion with all new lines, prop valve, and etc) using parts you can get anywhere, would be anything but a positive.

However I don't want to sell @HotRodLincoln short on his point that a badly done conversion would be unsafe.

Then again, I see these folks in the Skoolie forums making their buses up to 18" taller, with wildly variable welding and engineering skillsets behind those roof raises, and I think that adding a modern day master cylinder would be a lot more doable of a conversion/update.