Just did the brakes on my 57 pontiac, upgraded to a dual m/c 1" bore. took awhile but I got the pedal travel/pushrod length all correct. Bled the brakes, about 4 times, adjusted the shoes. Now the m/c is under the floor, I used 2# residual valves(I know they should be !0#). my problem is I have no pedal pressure, or very little, I find it hard to believe that just using 10# valves is gonna fix this. Is there something else i'm missing here?? Shouldn't I have some brake pressure even if I didn't use residual valves? Oh its a drum/drum set up.
Do you have a bit of freeplay before the pushrod touches the M/C piston? That will make it tough to bleed right.
Then it must be a lot of air if no pedal. How I bleed underfloor or firewall systems; I just open a bleeder and slowly work the pedal till I hear a flow. Then tighten and do the next. Just don't let the pedal fly up, go slow. I can't think of the last time I needed somone in the car; this usually works for me anyways.
I know, thats what I keep thinking, gotta be air in the lines. I bled the system atleast 4 times, i got nothing but a steady stream of fluid coming outta each bleeder, but the pedal still won't stiffen up.
Some good auto parts places have block off flare fittings, same sizes as flared brake tubing. If you just can't sort it out, maybe block off the fronts or rears to help figure it out?
Did you bleed the M/C? Some guys swear you have to bench bleed them. I have a 48 chevy with disc brakes and an under-floor set-up. Bled the M?C, opened the bleeders for a day, then put a mighty-vac on each one. They came right up.
With your early drum brakes, you really need residuals, if the master does not already have them internally. Air can be pulled past the wheel cylinder cups during brake release. Late 60s/70s drum brakes used metal cup expanders that prevented this. Why did you use 2 lb instead of the correct 10 lb valves? Install the 10 pounders. Is the new dual master bore the same as the '57 Poncho? Very important-did you check to make sure the pedal bottoms the master pistons before it hits the floor? (This check has to be made with both systems open) If the master cannot be full stroked, you'll never get a good bleed. When bleeding, it's best to use a piece of tygon (clear) tubing pushed on the bleeder and emptying into a container. (3/16" id is the most common) Have the tubing angle higher than the bleed screw, and you can easily see fluid and air coming out of the bleeder as someone slowly pushes the pedal full stroke, then closing the bleeder when fluid/air stop flowing and before the pedal is released. Perform this 2-3 times at each wheel, making sure the master reservoir does not get too low. When you have a good pedal, do a leak test by applying max pedal effort, making sure the pedal does not fall, and hold for 15-20 seconds. Check every system connection for weeps/leaks, and if all is dry, you done.
Along with what V8 Bob said about 10lb RPV's, I was taught that RPV's keep the springs on drums from totally collapsing the wheel cylinders (sending way too much fluid back to m/c). If all of this fluid goes back to the m/c, it must be replaced before the brakes will hit. This effect will cause a soft pedal at the least and usually cause brakes that need to be pumped up. Keep in mind, some m/c's have built in rpv's and you might be right about the air, but you MUST run 10psi rpv's with drums.
I am a believer of the must bench bleed the M/C group. I also like my Mityvac. The reverse bleeder systems on the market work very well too, as well as the good old fashion get your buddy/wife to pump and you crack the bleeders system. 10# valves with the drums.
This is a common belief, but is totally wrong. A 10, or higher factory 15 to 20 lb residual can in no way overcome powerful shoe return springs, with around 60-100 lbs or more force, depending on brake size, design, etc. The shoes are are pulled back to, and rest, on their anchors upon brake release by the return springs. The cylinders cannot collapse because the anchors control the shoe's position at the wheel cylinders. Because of the strength of the return springs, air can be pulled past the cylinder cups during fast brake release. The residual creates a checked low pressure that prevents/limits a vacuum, and reduces displacement that results in a "tighter" system with minimum pedal travel loss during brake application.
Why did you use 2 lb instead of the correct 10 lb valves? Install the 10 pounders Unfortunately my parts guy gave me the wrong info and sold me 2lb valves, now I know that they're wrong i'll be picking up the 10lb and try them. Hopefully this will be the solution.
I don't believe RPV's in the system or none ,during a short test, would effect the outcome a bit. I'm wondering about a bad M/C. may be building enough pressure to bled out an open bleeder but not enough pressure to give a good pedal.
""Because of the strength of the return springs, air can be pulled past the cylinder cups during fast brake release. The residual creates a checked low pressure that prevents/limits a vacuum, and reduces displacement that results in a "tighter" system with minimum pedal travel loss during brake application."" Residual valve mainly allows the brake mc to back fill the system [i.e., pumping] should there be a problem, like drum brakes needing adjustment...10# for drum brakes regardless where mc is, a necessity..With the use of disc brakes and under floor mc it [2# residual] is necessary for anti drain back value but probably allows some mc back filling but disc brakes by design maintain "adjustment"..
Unless that M/C was designed for drum/drum brakes it could be a volume problem in the M/C. Went through that a month ago.
seb, I'll stand by my explanation of residuals. They have nothing to do with back filling the m/c, or helping with brakes out of adjustment. I do agree, partially, that residuals are a necessity on older pre-early 70s drum brakes, and can help later brakes with cup expanders. I also agree that 2# residuals should (only) be used on under floor disc masters. Any pressure much higher than 2# will likely overcome the caliper seal and cause piston movement and/or prevent the seal form retracting the piston, resulting in a "runaway" brake. Residuals were never used on modern disc-equiped production vehicles because the masters were always high on the firewall.
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