hey guys i just converted my 53 chevy pick up over past few weeks to manual disc using newer corvette rotors and calipers front and rear. the truck had a tandem master bendix style for the 4 wheel drums all around i think the bore is 7/8 now with the larger discs on the truck it stops great but the pedal feels mushy and very soft and there is just too much travel. i bled the system many times it isnt air i think the master needs to be upped in size. i called around and spoke to SSBC< CPP and Wilwood and got all different answers from all 3. they tell me i need a 7/8 which i already have, others say 1 inch. Wilwood suggest i try a 15/16. or a 1 inch can any of you guys point me in the right direction so i don't have to do things 2-3 times over to get it right
Is it mushy to the floor? A bigger bore master will make the pedal harder to push, meaning more leg effort for more volume with the same clamping force not necessarily loose the mushy. Mushy is air compressing or something else expanding.
1 inch is usually about right for manual drum brakes, here's a reference (slightly older corvette back when you could get manual brakes) http://www.rockauto.com/catalog/x,carcode,1040892,parttype,1836 Won't fix the mushy....
I've owned a 69 Corvette with manual brakes for 45 years. Up until the time when I put stainless sleeved calipers on it, I probably did more brake work on it than all other work combined. The 69 GM service manual says you will not get all the air out of these brakes unless you bleed them with a pressure bleeder, and the manual is 100% correct. With all the air out of the system, and with the stock 1" bore master cylinder, the pedal is firm and the travel is right where you'd want it to be. The brake feel is much better IMO than it is on the same car with power brakes, which tend to be a little too touchy for my taste. Because the M/C displacement varies as the square of the bore size, a 7/8 bore cylinder only displaces slightly more than 3/4 of the fluid a 1" bore cylinder will move for the same travel. If you're using the calipers and rotors as used on Vettes up thru 82, the 7/8 bore M/C is going to give you too much pedal travel.
More travel but also increased pressure. If you increase the MC size to 1 inch you will get less travel, but also less pressure, requiring increased effort on the brake pedal by you. Like already stated, none of this has anything to do with the "mushy" feel.
guys the brakes are from a C5 vette i was told, i think piston area is about 4 inches front and 2 out back i used a motive power bleeder to bleed the system 3 times so there isnt any air i just think the 7/8 i had been using for my drums just isnt a good match for the new brakes. i am leaning toward trying a 1 inch bore just wanted to hear some of your feedback with this. the pedal now just feels spongy and according to a few companies i spoke with it suppose to feel this way because it makes more pressure and less volume so maybe i need more volume and a tough less pressure i certainly dont know
Disc calipers do need more volume than drum. So you are thinking right about going up in size. As stated, all else being equal, the pedal force will be higher for same line pressure. Pressure is what does the work, not volume, although you need enough volume so the pedal does not go to the floor. I think 1 inch is a good starting point. 3/4 inch is 0.442 sq inch of MC piston area. 1 inch is 0.785 sq inch of piston area. Or in comparison, the 1 inch MC is about 78% more volume.
if its a dual side corvette master you have to loosen the caps from the non used side and let the fluid trickle under pressure as it will hold air and give a spongy feel to the pedal as it traps air in the ports and drillings
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