I have put in a disc conversion from Chev's of the 40's into a friend's 50 Chevy. Also used their dual master cylinder kit. Everything fit OK, but now that it's done, the car does not have the stopping power it should. Bled it every different way I know, regular, gentle, reverse, with a vacuum pump, and with a power bleeder (this worked best). I get a pedal that goes halfway to the floor, no sponginess. But stopping power is weak. So, trying to figure out what's wrong. Master is a 70 Mustang, dual, for front discs. 1" bore. Calipers are 73-76 Monte Carlos, as well as 70-77 Camaro and others. I tried to measure the pedal ratio (master is under the floor, so it's difficult to get precise), and it's around 4.1 to 1. Doesn't seem like it will work like that. So, you math whizzes, will a bigger bore or smaller bore give me more stopping power? I hate to try to change the pedal ratio, it's the stock pedals, and they fit nice. Any suggestions?
The smaller bore will give you more pressure, but at a loss of pedal height when the brakes are applied.
4:1 isn't enough, you're right. Smaller bore will do as Doublepumper says...May have to go a fair amount smaller and that could be hard to find. The math will tell the size to use. Going from a 1" to 7/8" helped my Fairlane a good amount. About 3/4" is what you'd need to get close to 1000 psi with 100 lbs of leg force.Probably hard to modify the under-floor pedal to master lever length (make it shorter from the pivot)? Here's a calculator- http://www.wallaceracing.com/brake-master-cyl-calc1.php#google_vignette
Not anywhere near enough pedal ratio, needs to be 6-7:1, as said a smaller master diameter will help but with increased pedal travel.
Your pedal ratio is higher than that. Lemme dig up some numbers out of my brake book after I am done BBQing
Other things to check, are the calipers on the correct sides so that the bleeders are at the top. Does the master cylinder push rod have 1/16" free play? A smaller bore master will help as said.
Use a PBR VH40 inline [remote] booster on the front circuit. [the output pressure is up to 1470 psi] Then you don't have to dick with the plumbing or pedal ratios. With a manual brake you want a 7/8" bore m/c stepped bore from an S10 If you don't want a booster and the pedal ratio is too high , consider a 3/4" stepped bore M/C of a 70's Mercedes [the stepped bore takes care of the initial pedal travel]
So far everything I've double, triple checked is OK, bleeder up top, tiny amount of free play, Master getting full travel underneath, no leaks anywhere, brand new hard lines and hoses everywhere. Not my first rodeo. In fact, I tend to get the really difficult problems from my friends, and fellow car guys, as I am relentless in finding problems and fixing them. So, since it is very inconvenient, and costly to my friend/customer, to change pedal ratios, or calipers, it looks like the master is the solution. I've already changed it out once, to the same type (made in China, so I thought it would be the problem), but I dont' have an issue with changing it out again, not charging for time, or re-terminating brake lines, but I want to be sure it will work. Time is a'wasting, and he wants to go to Lead East. Will a 7/8" one do the trick, or is it safer to go with a 3/4" stepped one (if readily available. Really galls me that it was a complete conversion kit, from one source, it doesn't work right. I know they get it from a vendor, because I called them, and they have no technical line to talk to.
Quick question, before I run to the store (to get an S-10 master, also stepped bore). Will I have to buy a new flaring tool to do the metric flare? My Rigid one only does the old style double flare.
I have a Wilwood front disc system. No vacuum assist. 7/8” MC. 5-1/2 to 1 pedal. Works fine better than 4 drums. Calipers are 4 piston which may make a difference. If you’ve changed the rear end you need to make sure the wheel cylinder are not from a power brake vehicle. Good luck.
Finally fixed! The stepped bore master cylinder did the trick. I used one out of 1985 S10 pick-up. While researching the Mercedes one, the GM one was mentioned several times. Had to cut a slightly larger hole in the floor, and do a bit of grinding on the master to fit, but a quick bleed, and we had brakes. I thank you all, and Teddy thanks you, too, now he can drive to Lead East!
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