Over the years (2-3 decades ago) if you were working on a heavy truck with juice brakes and the brakes were doing funny things, first thing you did was replac the hydrovac and the issue usually went away.. Thankfully Hydrovacs are a thing of the past. Unfortunately so are replacement parts....and the people who knew anything about them are becomming hard to find....
So on to my current dilemma... Brought home a Coleman 4x4 Tug Tractor, 4 wheel steer rated for 130,000 pound air plane spotting. Will go about 25 miles an hour with a good tail wind. Had been sitting a few years after being rebuilt by a retired aircraft mechanic. I put brake fluid in the small fluid reservoir and let it set a couple weeks. Pushed the pedal a couple times and got a hard pedal that felt right... Started the engine and with the vacuum assistance pushed on the brake pedal a couple times, felt right then on the 3rd push, pedal went to the floor.. Pumped the brakes till the reservoir was dry and looked underneath for a leak. Nothing.. Repeated this process till I had pumped through 2 quarts of brake fluid and find no leaks underneath.. Fluid must be in the hydrovac...?
Currently there is a passenger car size vacuum booster on the firewall and a small single reservoir master cylinder with one brake line coming out of that down to the hydrovac bolted to the frame. I am assuming the function of the hydrovac is to send a large amount of fluid out to the wheel cylinders when signaled by the small master cylinder roughly figuring? The odds of finding a replacement hydrovac would likely be slim...
My question- I look at more modern high gvw trucks in the boneyard with juice brakes and they have LARGE vacuum boosters on the firewall with LARGE master cylinders with double reservoir. This is a slow moving off road vehicle that will never again be towing a 130,000 pound aircraft unassisted. I'm just gonna plow my driveway with it. Just need some brakes... Anybody have any thoughts of the success of stripping out the larger vacuum booster and master cylinder from the modern high GVW truck and having enough fluid volume movement capacity from the large master cylinder to equal the hydrovac and make the wheel brakes somewhat functional? I won't be attempting to stop any 130,000 pound aircraft... Recognized the large modern master cylinder on the 80's GMC 6500 I'm looking at as a donor vehicle is double reservoir and I"d have to take the divider block from the donor truck and feed each axel from the divider block. Am I on to a possible off road fix or completely looking down a path to wasted time/failure. What do you think guys?
