Ok here's the Story.. I have a 49 Plymouth on a GM Full Frame.. (Which I am as yet unable to Identify model and year, Any help there would be greatly appreciated) It has Drum Brakes on all 4 and I am planning on upgrading to Disks if I can at some point determine what my frame is.. Ok I'm Rambling.. Have crate motor, PS, PB, AC etc.. Brake setup is off something with disk brakes, dual reservoir, 11 1/2" Booster.. Issue is the brakes are so touchy that if your not really careful you end up with your nose on the windshield.. I am aware that I am over boosting the Drum Brakes.. The would usually require 500psi Boost and the booster I have puts out 1000psi that's required for Front Disks. Question is... Will installing a smaller size booster, Like a 9" help with this? Or does someone make an adjustable metering Valve? or? Or do I live with it until I can identify the frame and get my Disk Conversion?
lifting the world with a lever 9 miles long, will take more effort than one 11 miles long. lol.. sure 9 will be better( less push)
What is the MC bore size? A larger bore MC will give lower line pressure,if you have say a 1" going to a 1-1/8 will help. If it is already 1-1/8 a smaller booster is all you cand do.
It's an 1-1/8 already.. I bought a smaller booster, And I guess I will install it when I Rewire the car as I am planning installing a new Steering Column then as well.. I'll let you know how it works! Now.. Who's the "Frame Guru" Round here?
you may try restricting the vacuum some now....drill a dowel with an orfice n insert in the hose. no money ,not much work. giv'er a try
Is there quite a bit of travel in the pedal with no effect, then they just suddenly want to grab? Maybe the proportioning valve needs to have a check valve (residual pressure valve) with it. Do these brakes "cross over"? (LR same hose as RF, RR same hose as LF?)
What kind of pedal ***embly do you have? What is the pedal ratio? A long pedal arm that gives you lots of leverage may mean you don't need a booster. The master cylinder doesn't know or care whether it's being boosted by a vacuum booster or the mechanical leverage of a long pedal arm.
LOL...oldolds, has the best first try.STOP OUT THINKING THE PROBLEM. Try OLDOLDS IDEA, unhook vacuum line , pull out fitting(important) at the booster.... plug engine vac side or pinch hose with vise grip, and give it a try, may work without power. or reduce vac supply like one suggested.
Yep - to beat a dead horse - you have way too much booster for drum brakes. I would remove the booster totally - but you may have to increase pedal ratio ( you probably have 3.5:1, need about 6:1 ) and/or decrease master cyl size if pedal is then too hard. Might be easier to just do the disc brake conversion - but you will still have to play with sizes and ratios depending on what wheel brakes you use. Brakes aren't "one size fits all"........
I had this same problem, the remedy is to replace the slave cylinders with a smaller ones, we replaced a 1 1/8” cylinder with a 1” front and a 15/16” with a 7/8” rear on a jeep dispatcher. Unless you can physically look at them at your local parts store it can be difficult to find the right number. If you know the original cylinder number I may be able to figure the corresponding smaller cylinder numbers.
The Jalopy Journal
