Discussion in 'Brakes for your Classic Mopar' started by khuebner250, Sep 3, 2018.
That would lead me to believe there's some kind of master cylinder problem going on
If you hold the pedal down, it will sink over time if you have a bad master cylinder. I would block the pedal down overnight and then re-bleed in the morning. Usually if you hold pressure for a long period, it will work the air out.
We are on the same page.
Did you ever solve this? I’m having the same issue with a similar setup
I'm not sure... Cass sent me a new master cylinder. Although better, it's still not great. My car isn't driving yet though. So I'm not really sure. It'd be nice to drive it and rebleed after it's had some heat and drive time on em. Thanks, Keith
You have 4 wheel disc brakes and a 15/16" master cyl, although it gives you more stopping power it takes more travel to move all the pistons. I think you would be better off with the 1 1/32" master cyl as it will move more fluid and not travel as far. Power booster would help a lot, the ratio is different and you can use a bigger master cyl.
There is nothing in a disk system that will "back the pistons out" except the deforming lip on the piston seal with relaxed pressure and that will withdraw the piston about .002-008 and that is by design. Thats all the pistons are going to move under pressure minus the caliper and line flex. You can disco and plug the rear circuit (one plug) and start bleeding from there. Should tell you what curcuit has the air. If you get the fronts rock hard, you know the rears will just need further bleeding to get a hard pedal. to bench bleed the MC just loop the 2 outputs back into their reservoirs under the fluid line and pump away until no bubbles are seen. Mighty vac on each bleeder will assist also, just make sure you dont run a reservoir dry.
A 15/16 master has higher line pressure. Should result in less pedal effort and greater feel.
3 psi residual valves?
If the rear calipers are equiped with an internal emergency brake (ratchet screw) behind the Piston, the emergency lever may need to be cycled several times to get the pads/pistons into contact with the disc.
Cass from Dr Diff sent me this link trying to help me get my pedal to feel right. It made a lot of sense and I just knew it would fix the issue but nothing changed. Maybe it’ll help some of you. I think my new master may be defective.
Yep, that's the one, You won't get a hard pedal till you adjust the emergency brake. Thanx Craig. Try bleeding with the emergency adjusted AND applied. Cheers
This is true but you really don't know how much pedal effort you have till you drive it and the smaller the master the more travel you have.
Well shit, you might be on to something there! My ebrake isn't hooked up, so that hadn't even crossed my mind. I'll have to give it a go.
Yea sometimes you have to pump it a lot of times to get the ratchet to catch inside the pistons, they should have that in the instructions.
72bluNblu what do you disagree with?
Everything in your statement. The 15/16” master should work fine. The 1-1/32” isn’t a better choice in my opinion (I’ve run both). The long travel in this case isn’t an issue with the master cylinder diameter. And the power booster comment is wrong. Yes, there is a linkage difference, but it doesn’t work the way you think it does. The factory ran 15/16” masters with the power booster and 1-1/32” masters with manual brakes.
Also what does your car weigh, a mustang gt is around 3800lbs, a hellcat challenger is 4450 lbs, I'm sure you have 1000-1500lbs less on them.
That is what I said, the smaller master with power brakes because the ratio is different and the master moves further and the pedal less than with manual brakes, that's why you need a bigger diam master with manual brakes to fill the 10 caliper pistons with fluid, and if you add a booster it wont take very much pedal effort at all to stop the car. But like I said it is a lot of brake for a light car so he shouldn't have a problem.
I think there may be confusion over which m/c will fill the Piston Chambers more quickly, - as opposed to which m/c will move up to 10 pistons of "x" diameters 4 or 5 thou. with the greatest "mechanical" advantage. Once the servos are full, only a few CC's of M/C fluid is needed to apply full brake with minimal pedal movement, regardless of M/C dia. Cheers
That's not really right either. Pressure=Force/Area, that's all you need to know here All things being the same: A smaller master has more pedal stroke to deliver the same amount of fluid and therefore will feel softer In an extreme case a smaller master may not be able to deliver the necessary fluid to stop the car without bottoming In the case of the A-bodies, all the pedals have the same ratio, it's true there is a fulcrum on the booster that changes the ratio and therefore reduces the travel. In general you want a bigger MC with a booster. In my case, manual brakes, you want a 15/16", your braking performance will be better. I actually have hydroboost and have a 1 1/8" master cylinder on the car since it has a ton of boost.
Separate names with a comma.