Power brake master question

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Oct 17, 2013 | 10:33 AM

68fish

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Power brake master question

Are there different sized piston diameters for power brake master cylinders?
I just swapped out my brakes from non power drum to power disc and I have way too much pedal travel for my liking. I bench bled the master and bled the system twice to be sure it's not an air issue and the pedal still travels half way to the floor before engaging and bottoms out more than 3/4's of the way down. With the old brakes I had to push hard but no way was the pedal going more than half way down. I'm thinking I might have a master with a small piston diameter requiring more pedal travel. It's a rebuilt unit listed for a 1970 F100 2WD w/ 302 V8.
Thanks for any help, after doing all that work I was hoping for a better result. The brakes are still far better than drum though so if this is something I'm just going to have to get used to I guess I'll just have to get used to it.

Oct 17, 2013 | 10:49 AM

FECruzer69

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It takes more fluid to apply discs than it does to apply drums.
Does your Master Cylinder have two equal size reservoirs or does it have one large reservoir and one small reservoir?

Oct 17, 2013 | 10:52 AM

68fish

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It's got the standard small up front for the rear brakes and large in the back for the front brakes. It's physically a bit larger than the drum master. Bolted right up.

Oct 17, 2013 | 11:00 AM

FECruzer69

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OK. Generally the vehicles with front disc brakes use the master cylinders with the large reservoir for the front/small reservoir for the rear. When you swapped from drum to disc did you take the combination valve/proportioning valve from the donor truck too?

Oct 17, 2013 | 11:09 AM

68fish

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I left my valve as is. That shouldn't effect pedal travel though, only distribution theoretically. That would be a subject in itself! My friend converted his from drum to disc (non power though) and left his valve alone as well and had no issues with the rear brakes locking before the fronts. That would be the proportioning valve's only function other than triggering the brake failure light.

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Oct 17, 2013 | 10:57 PM

orich

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From: **** hole San Jose ca.

Some masters have a 1" & some are 1.1/4" that, I've seen listed for the 70's= 1" and
some 73-79's had 1.1/4 From what, I've read if master piston it to large for the system
peddle can be spongy and not a hard peddle. I don't know if the single piston calipers uses more fluid then the dual piston. Some master cyl have the size of the piston on the bottom of them..
orich

Oct 17, 2013 | 11:14 PM

NumberDummy

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From: Simi Valley, CA

No F100/150 had dual piston calipers.

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Oct 18, 2013 | 05:41 AM

yetiman38237

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From: Mayfield, KY

If you decide you do want a prop. valve I have one......came mounted to the top of steering box and I already had one.

Oct 18, 2013 | 06:10 AM

#10

NumberDummy

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From: Simi Valley, CA

D3TZ-2B257-A .. Brake Pressure Differential Valve-Use with disc brakes / Marked: D3TA-2B524-AA

1973/74 F100 2WD / 1973/74 F250 2WD with single piston calipers.
-----------------------------------------------------------------------------
D5TZ-2B257-B .. Brake Pressure Differential Valve-Use with disc brakes / Marked: D5TA-2B524-FA or FB

1975/79 F100/150 2WD / 1975/79 E100/150 / 1975/79 F250 2WD with single piston calipers / 1976/79 F100/150 4WD / 1976/79 F250 4WD / 1978/79 Bronco / 1979 F350 4WD

Oct 18, 2013 | 10:06 AM

#11

68fish

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Quote:

Originally Posted by yetiman38237

If you decide you do want a prop. valve I have one......came mounted to the top of steering box and I already had one.

Thanks for the offer,

I'll know by the weekend I hope.

Oct 19, 2013 | 12:54 PM

#12

68fish

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The only marking cast into the master is a "C" on the bottom. Everything else is plain wrapped. So it looks like it is what it is and I'll just have to get used to it. Thanks all for the help.

Oct 19, 2013 | 08:13 PM

#13

orich

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From: **** hole San Jose ca.

Looks like it's suppoes to be a 1" piston bore from what is listed on the auto parts sites.
You would have remove the bolts on the MC measure the bore opening to be sure..
That pix of it looks to big for a 1" MC more like a 1.1/4" IMO ..orich