Brake Pedal Travel?

[MiraclePieCo]

I'll post this here since I have a '68 GM 12-bolt rear in my truck.

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?
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1951 Chevy Panel Truck

[geezer#99]

How did You lengthened the pedal arm?
Did you change the pedal ratio?

[mgchevyparts]

obviously your booster is working. It's the master bore diameter that determines the pedal travel, well technically the bore diameter in relation to the caliper bore sizes. I would guess you might have a 1" bore master, changing to an 1 1/8" master will bring the pedal up an inch or maybe a little more. The smaller the bore of the master the higher the pressure BUT less fluid is moved. The larger the bore the pressure is lower but the pedal doesn't have to travel as far to move the same amount of fluid. The only time you use a small bore master is if the caliper pistons are really small. Since you have Mustang II I would bet you have S10 calipers which have 2.5" pistons. That size is borerline between the two master sizes. Since you don't like the way it is now change to a 1 1/8" master and it will come up and will be a little firmer feel also.

[MiraclePieCo]

Originally Posted by geezer#99

How did You lengthened the pedal arm?
Did you change the pedal ratio?

Yes, I lengthened the pedal arm about two inches to get it into a more comfortable position. No doubt that added slightly to the pedal travel.

Originally Posted by mgchevyparts

obviously your booster is working. It's the master bore diameter that determines the pedal travel, well technically the bore diameter in relation to the caliper bore sizes. I would guess you might have a 1" bore master, changing to an 1 1/8" master will bring the pedal up an inch or maybe a little more. The smaller the bore of the master the higher the pressure BUT less fluid is moved. The larger the bore the pressure is lower but the pedal doesn't have to travel as far to move the same amount of fluid. Since you don't like the way it is now change to a 1 1/8" master and it will come up and will be a little firmer feel also.

Yep, my MC is a 1" bore so I think you nailed it. Now I'll have to consider if it's worth changing MCs...

[geezer#99]

Originally Posted by MiraclePieCo

Yes, I lengthened the pedal arm about two inches to get it into a more comfortable position. No doubt that added slightly to the pedal travel.



.

You need a pedal ratio closer to 4.5 to 1.
An easy test is to drill a new pushrod hole in the pedal about 1 1/2 inches further from the pivot point.

[67ranchhand]

Originally Posted by geezer#99

You need a pedal ratio closer to 4.5 to 1.
An easy test is to drill a new pushrod hole in the pedal about 1 1/2 inches further from the pivot point.

Not to steal the thread, but how do you figure out pedal ratio? And which way would the op move the pivot point? I may need to do this on my 51 3100 and thought I would ask. Maybe it will help someone else too.

[geezer#99]

Try this.

https://techtalk.mpbrakes.com/how-to...ng-pedal-ratio

[MiraclePieCo]

Originally Posted by geezer#99

Try this.

https://techtalk.mpbrakes.com/how-to...ng-pedal-ratio

While checking my pedal ratio (4.7) 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.

I was looking for zebras when in fact it was just horses.

[68 P.O.S.]

Stock bore size