Brake pedal problems -disc/disc

[Topsterguy]

A friend has a 34 Ford tudor and he's having ongoing problems with his brakes. He's running
8" dual booster
Corvette style 1" master cyl
proportioning valve Disc / disc
Chev Malibu front calipers
'98 Mustang rear calipers on 8.8"
He measured the pressure and he's getting 600psi on the front and 200psi on the back
The complaint is he's got low pedal with the engine off, then it goes almost to the floor when it's started, and overall it's a very soft brake pedal. He says he's done all the bench bleeding etc etc, but can't solve it. :?

Anyone got ideas !!???

[wayne petty]

any chance of getting the distance from the center of the pedal pivot to the center of the master push rod attachment..


and possibly how much the actual travel the brake master push rod moves  at full stroke? maybe just putting a mark on the pushrod with a sharpie and pushing the pedal and putting another mark...  then measure between them...

if the pedal ratio is off there will not be enough push from the pedal to move the pistons out ...

[kb426]

My vote is for air in the system some where, some how. That's what's most common for spongy pedal.

[Topsterguy]

any chance of getting the distance from the center of the pedal pivot to the center of the master push rod attachment..


and possibly how much the actual travel the brake master push rod moves  at full stroke? maybe just putting a mark on the pushrod with a sharpie and pushing the pedal and putting another mark...  then measure between them...

if the pedal ratio is off there will not be enough push from the pedal to move the pistons out ...

Thanks Wayne!  I passed this on to him and he's wondering what the right amount of travel would be, and how he can find out if what he has is the right amount?

[Topsterguy]

My vote is for air in the system some where, some how. That's what's most common for spongy pedal.

He's positive that there's no air in there - the first thing he did was pressure bleed it completely.

[enjenjo]

For the master cylinder, the correct amount of travel is from where the pistion sets at rest, to it being bottomed out. Normally somewhere between 1.5 and 2".

Also, there are two places to check for free play, from the pedal to the booster, and from the booster to the master cylinder.

His pressures sound way too low, I would think he would be seeing pressures in the 700psi to 900psi range

[38HAULR]

I would have a stab at the actuating pin between the booster and the m/c being too short ........Frank.

[Topsterguy]

As always, thanks BIGTIME  guys! I'll pass this on to him!

[enjenjo]

I just replaced a master cylinder today, and found the booster to master actuating pin was misadjusted. More common than you might think.