Car 539 Ė Upgrade to add a brake booster resulted in an overly sensitive brake pedal. See the PDF file on this project http://www.glassicannex.org/tech/chassis/brake-booster-car539.pdf and then read this followup info by the owner after he solved the problem.

 

Proportioning Valve

When I completed my upgrade to power brakes I noticed, that the brakes were terribly touchy. Just a slight touch on the brake pedal and it would throw you through the windshield.

First I shortened the brake pedal to get the proper brake pedal ratio of 4-5:1 (manual brakes have 5-7:1) but that didnít change a thing so I used my original brake pedal. BTW, the brake pedal assembly used in our Glassic is Ford Bronco 1966-1977.Here is an interesting article about brake pedal ratio.

http://knowhow.napaonline.com/how-to-measure-brake-pedal-ratio/

 

I finally resolved the issue with the touchy brake by adding an adjustable proportioning valve between master cylinder and pressure differential valve to prevent the rear brakes from locking up.I used a Wilwood valve Part # 260-8419 and reduced the pressure the max 57%.

 

https://www.wilwood.com/MasterCylinders/MasterCylinderValves

 

Brakes work perfectly now.

Note: If you donít need your brake warning light, you could also, instead of adding the proportioning valve, replace the pressure differential valve (where the sensor for the brake warning light sits) and replace it with a Combination Proportioning Valvepart # 260-11179. It could also function as a splitter for the two front brake lines.