Posted by Greg Coffin [172.68.35.84] on Tuesday, March 26, 2024 at 16:12:52 :
In Reply to: Disc brake conversion posted by jay massy [172.71.158.81] on Monday, March 25, 2024 at 16:43:46 :
My guess is the problem is a non-functioning vacuum booster. The 1-5/16" master cylinder is a bit oversize, and it could also be contributing to a hard pedal. But I think the booster is your main problem.
Test the vacuum booster by pumping the brake pedal a few times, then hold the pedal down while you start the engine. If the pedal drops a bit when the engine starts the booster is working. If not, the booster isn't working. Another way to check the booster is to pull the vacuum line and test the brakes without boost. If there's no change then the booster is the problem
If the booster isn't working, check the vacuum line at the booster for vacuum, and possible vacuum leaks. Does it have a check valve at the booster, and is it going the right direction?
37" tires are just a little over stock, so I don't think that's the problem. Disc brakes are what you want on a heavy truck. They are far superior to drum brakes, but they take a lot more line pressure to make them work well.
For reference, my brake kits use a 1-1/4" master cylinder and 7" single diaphragm vacuum booster. The system works very well with original drum/drum brakes, and upgraded disc/drum and disc/disc brake systems.
Let us know how things progress.
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