Problem I'm having isn't an excess of pedal travel as such. During driving the pedal travel is what would be expected. It's just than when I come to a halt if I keep my foot on the pedal it starts to drop away.
Am I underestimating the Hydroboost pressure applied and it is causing some physical change eg, balooning hose etc? The force on the pedal when this happens would pull the truck up much quicker during driving than the lighter force I do use.
IF (big IF) there are no external leaks, and IF you are not applying more pedal force once stopped...a slowly sinking pedal is classic symptom of an internally-leaking master cylinder. In other words, you need a new master cylinder that works properly.
If you are increasing pressure on the brake pedal once stopped, having the pedal sink a little bit
and then hold firm is totally normal, for all the reasons given in a previous post--caliper flex, hose ballooning, seal distortion, heck, even firewall flex can make a difference in perceived pedal height.
If the pedal continues to sink...the master has failed.
The flaw in your theory is you assume a fixed applied pressure at the pedal. While the pedal effort will increase you can apply more pressure with your leg/foot and achieve the same presure at the caliper piston.
I guess.
I can
maybe apply increased pedal pressure. YOU can perhaps apply increased pedal pressure. The Original Poster, and everyone else who's ever completed this "mod" can, perhaps, apply increased pedal pressure.
They (we) shouldn't have to. And my knees aren't what they used to be.
Can their sister, mother, teenage kid, valet parking attendant, or the NEXT OWNER of the truck, apply that much pedal pressure?
At least one guy removed his GMT800 big-bore master cylinder because he found that in a panic stop, he could NOT apply enough pedal pressure unless he used both feet and a squirt of adrenaline.
The GMT800 big-bore master cylinder "mod" is NOT SAFE particularly if installed on ****** little JB3*-style power boosters. It's bad enough on the larger JB5--6 boosters. Folks are using the GMT800 big-bore master cylinder to crutch another problem in their vehicle's brake system.
Essentially, they're "fixing" the wrong thing. They should be adjusting (or upgrading) the rear brakes, removing air from the system--abs valves, or whereever--replacing sticky/seized calipers, replacing glazed or low-friction pads/shoes, or replacing a failed step-bore master cylinder with a properly-functioning step-bore master cylinder.
*Note that the JB3 brakes were so ****** that GM quit using them in the early-'90s. My 88 K1500 came with JB3 brakes, I've since converted to JB6--but the big change was removing the "10-inch" drums for the 11.15" duo-servo drums.
Here's the JB3 booster (used with 1" bore master cylinder) compared to a JB5--6 booster (used with 1 1/8 bore master. ) Bigger master cylinder bore provides less hydraulic advantage, needs stronger power booster to maintain moderate pedal effort.
Photo 1. JB5 booster (left) and JB3 booster (right).
http://hbassociates.us/K1500_2020_Brake_Booster_02.jpg
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Must be a truck thing. The Suburbans got the 11" rear drums too, they're a known junkyard bolt on. I don't know if they're leading/trailing or duo servo though as I've never laid eyes on one personally.
So far as I know, the ONLY leading/trailing shoe rear brakes on GMT400 series vehicles are the "10-inch x 2.25" drum units.
Officially, they're 254mm x 57mm, and the drums have a metric measurement cast into them. They're only used on JB 3 and JB/JD 5 brake systems. From JB/JD 6 and higher, you get Duo-Servo drums of increasing diameter and width, or rear discs.
There may--or may not--be a difference in the wheel cylinder diameter on the leading/trailing shoe rear brakes between JB3 and JB/JD5. I haven't looked into that.
