47-55.1 Brake Pedal Travel?

[MiraclePieCo]

7" POL dual chamber power booster, Mustang II front discs, GM rear drums.

With the engine off I have just 1 inch of pedal travel. With the engine running I have about 3 inches - about half way to the floor. This seems like excessive pedal travel - is it? I have bled the MC, bled all four brakes, adjusted the rear drum brakes, and adjusted my booster pushrod. Brakes work great, had to lock 'em up recently when a deer jumped out in front of me. Since there's a big difference in running/not running pedal travel I suspect the issue lies in the booster? I did lengthen the pedal arm a couple of inches, but I wouldn't think that would account for that much difference. What am I missing here?

[1project2many]

I would not say this is excessive for a disc/drum system.

What many folks including seasoned mechanics don't catch is that pedal height, when you're sitting in the shop, is really based on what you feel is the right amount of pressure applied to your leg by the brake booster while pedal height while driving is much more dependent on how quickly the vehicle is stopping.

There are two tests I usually use in this situation. 1) walk away from the vehicle for a period of time and forget about the way the pedal feels. Then get back in and repeat the test to see if alarms go off in your head... "Holy smokes! That's bad!" 2) If the first test is a pass, drive the vehicle. If it's stopping well with the pedal at a greater height than what you were measuring in the shop, the pedal is fine. No further worries are necessary.

[nvrdone]

im dealing with the same issue right now. only my setup is drums front & rear. after much headache, I stumbled on some interesting info. my m/c uses a 13/16" bore rear wheel cylinder but I have a 1" bore cylinder in the rear. so this weekend im swaping out the rear cylinders to 13/16". the info I found indicates that with a larger wheel cylinder the chamber doesn't have enough fluid volume to move the cylinder causing the pedal to drop. you might want to check the specs on your m/c & see if you have the correct wheel cylinder.

[geezer#99]

Originally Posted by nvrdone

im dealing with the same issue right now. only my setup is drums front & rear. after much headache, I stumbled on some interesting info. my m/c uses a 13/16" bore rear wheel cylinder but I have a 1" bore cylinder in the rear. so this weekend im swaping out the rear cylinders to 13/16". the info I found indicates that with a larger wheel cylinder the chamber doesn't have enough fluid volume to move the cylinder causing the pedal to drop. you might want to check the specs on your m/c & see if you have the correct wheel cylinder.

I’d plumb in a 10 # residual check valve before i’d Change cylinders.

[jweb]

Since you extended your arm that may have changed the ratio, here's a link on how to calculate pedal ratio:
https://techtalk.mpbrakes.com/how-to...ng-pedal-ratio

I think 3" of travel sounds normal, that's about what I have.

[MiraclePieCo]

Thanks gentlemen, lots of good advice.

Originally Posted by jweb

I think 3" of travel sounds normal, that's about what I have.

Yes, the brake application actually feels quite good. It's just more pedal travel than my off-topic daily driver, so I thought it may be too much. The master cylinder/wheel cylinder disparity is probably a factor, and lengthening my pedal arm certainly didn't help, but it sounds like I'm close enough to normal parameters that if it's not broken I shouldn't try to fix it.

[Matt_50]

My daily driver has less travel than my truck. I used the MC and pedal assembly from my 84. It moves just like yours. An inch with engine off drops some more with engine on. Feels same as when I drove my 84. Haven't gotten it on the road yet but the pedal feels good.

[MiraclePieCo]

Originally Posted by Matt_50

My daily driver has less travel than my truck. I used the MC and pedal assembly from my 84. It moves just like yours. An inch with engine off drops some more with engine on. Feels same as when I drove my 84. Haven't gotten it on the road yet but the pedal feels good.

Thanks. You are confirming that everything is probably a-ok with my system. It's just different than my daily driver. The advantage to a increased pedal ratio is that, although the pedal moves a bit more, the pedal pressure is quite light.

Edit: while inspecting the setup I noted that there is another hole for the MC rod on the arm where I could increase the ratio and get more MC travel with less pedal travel.
Plus I found about 3/4" of free play in the pedal that I could adjust out.



.

[mr48chev]

It sounds like you have the system bled right so I doubt that is the issue. I think my OT car has about the same thing going on though. Stock system but it has a good solid pedal with the engine off and the pedal drops more than I like with it running and that is with 4 wheel disk and all oem pieces.