WOW Guess we better rethink a few things. For those pulling a heavy trailer a bigger diamiater master cylinder will definatly reduce applied pressure to the brake linings.It is a simple matter of ratio.A smaller diamiater cylinder will move a lower volume of fluid per stroke ,effectivly creating a ratio where as a given stroke of the pedel moves the wheel cylinder a shorter distance per inch of pedal travel,with more force(A basic law of hydraulics).A larger bore will move the wheel cylinder further creating a higher pedal (but having less pressure applied to the wheel cylinder) for the same force applied to the pedal in either size cylinder.
Darn this seems complicated for such a simple theroy.
Lets see -what we are trying to accomplish is to apply the brake linings against the drum at a high pressure -with less foot pressure exerted.This requires a ratio of some sort either created by mechaincal advantage or by hydraulic advantage.If we hooked a lever to the brake shoses we would want to move the pedal a long distance and at the same time move the shoes a short but adaquate distance,creating more pressure on the shoes-with less pedal effort.(kinda like 1st gear VS 3rd gear)
If we moved a cup of liquid (the Master cylinder)with 6 inches of pedal travel,we would create a higher volume with less Pounds per square inch of pressure.If we move 1/2 cup of liquid with 6 inches of pedal travel we would have less volume but more pounds per square inch -provided we exerted the same amount of pressure on the pedal.The more weight we need to stop the more pressure we need on the brake shoes.If we make too much pressure we can create a spongy feel just because of the distortion in the mechanical part of the brakes.
Yes a bigger bore will create a higher pedal but it will decrease braking power.
Yes we have sensitive brakes in our trucks,often a faulty RABS valve emphasizes brake adjustment problems, or any other problems we may have.
I personally feel from my own experience(I suffered with lousy brakes for a while till I came to the conclusion I had a faulty rabs valve(great minds think slowly)
. On the road with a heavy load one time I replaced 2 master cylinders in 1 day still no brakes for a quick fix I bypassed my rabs valve,using the origional bore I can stop my truck with a 9,000 lb load and a 6,000 pound trailer(15,000 lbs total) very comfortably if my trailer brakes are working as they should.(I have always disliked the power of my trailers electric brakes so my truck does more than its share.
My conclusion -keep the brakes prpoerly adjusted ,with a good rabs valve things will work fine.
By the way I still don't have a rabs valve but when I don't have a trailer on my brakes are way too much,I experience lockup in the rear at the drop of a hat.I hardly use the old truck much now and will have a new rabs valve soon for safty sake.
Jeff I think you too may be dealing with a faulty RABS valve also.Your symptions sounded so much like mine.Most of our trucks brake pedals will sink to the floor if we stop the truck and stand on the brake pedal,but in normal use they are fine(I even question if this may be a sign of early RABS valve failure.The larger bore will move more fluid ,helping mask the problem. Now with no RABS valve mine doin't sink at all.
Jim you are completly correct I hope I helped you further explain this.I feel it is a very important area to completly understand.
I hope I made sense. Please comment.
Thanks Mr Bill
