As per the lengthy thread posted here...:
http://gbodyforum.com/viewtopic.php?f=1 ... A2cbKMep3s
Finally solved my brake problems.
Context:
Installing a 4-wheel disc
Chevrolet Caprice 1989 master cylinder on a g-body. This is a first step before I change the rear axle and use disc brakes on the rear. Rear brakes are still drums right now.
Symptoms:
* Brake pedal has a very firm feel for the first portion of the travel
* Pedal becomes almost "blocked" for the second (deeper) part of the travel
* When driving, there's little stopping power to the front discs
* After a few stops, no more stopping power up front & pedal travel is clearly blocked . Almost feels like it's metal on metal block, but really, it's an hydraulic blocked.
* When purging the brakes, pedal travel is very difficult / very difficult to purge the front brakes after the rear brakes are purged.
Root cause:
* Front/Rear brake lines mixed up at the master cylinder.
* The first portion of the master cylinder sends a lot of brake fluid to the rear wheels
* Rear wheel brakes accept fluid but become full/saturated quickly drum brakes (they need less brake fluid compared to disc brakes), hence the blocking feel in the pedal
* As the car is driven, pressure rises naturally in the system and pedal becomes more and more difficult to depress
* As pedal becomes more difficult to depress, less oil is sent to the front wheels
* Front wheel brake appear to "fade" progressively.
* After leaving the car to rest, pressure lowers naturally and front brakes become effective again.
* Purging the brakes with the system this way, it will feel like your proportioning valve is sticking and blocking the front circuit (if your valve has an internal low pressure piston, which all GM valves have)
Solution:
* Swap the brake lines at the proportioning valve level, and purge.
HOPE THIS HELPS...!!!!
