Larger bore master cylinder

97strokerHD · Mar 29, 2015

Anyone know of a larger bore master cylinder that will work on the obs trucks? Planning on swapping a disc brake rear axle and want a little more fluid volume.

Peroni · Mar 29, 2015

You want one for the OBS F Superduty. Those trucks had 4 wheel disc brakes and use a larger master than the smaller F series with rear drums.

97strokerHD · Mar 30, 2015

I thought those had hydroboost? Will a master cylinder from an f super duty still bolt to my vacuum booster?

old man dave · Mar 30, 2015

I didn't find the Motorcraft BRMC-39 on Amazon, ebay or Rockauto. I think its been discontinued. Rockauto did have alternative brands available with the cruise or no cruise options Do a lookup for a 96 F-SuperDuty turbo diesel. The SuperDuty master cylinders are listed under brake parts.

Got4wd · Mar 30, 2015

97strokerHD said:
I thought those had hydroboost? Will a master cylinder from an f super duty still bolt to my vacuum booster?

No they won't. A sd one will but you will need to find a way to use the valve for the rear brakes. I am using a sd master but I have rear disc brakes.

morefuel · Mar 30, 2015

I'm not positive but i believe the 97 f250 have a larger master cylinder. And the hydroboost seems unnecessary to me. My brakes feel wonderful with factory vacuum and the valve on the side of the Master cylinder being modified.

m j · Mar 30, 2015

larger diameter = increased pedal effort
why do you want 'more volume'? are you bottoming the brake pedal with the current master? if not then you do not need any more volume

old man dave · Apr 1, 2015

m j said:
larger diameter = increased pedal effort
why do you want 'more volume'? are you bottoming the brake pedal with the current master? if not then you do not need any more volume

I guess Captain Obvious missed the fact that the OP intends on installing rear discs and also missed the posts about relocating the brake pedal attachment point.

Reading comprehension never has been one of his high points anyway.

old man dave · Apr 1, 2015

morefuel said:
I'm not positive but i believe the 97 f250 have a larger master cylinder. And the hydroboost seems unnecessary to me. My brakes feel wonderful with factory vacuum and the valve on the side of the Master cylinder being modified.

Same 1-1/4" master listed for all OBS years for over 8500 gvw F-250s.

I drove a '08 F-350 work truck and those brakes worked waaay better than any OBS I've ever driven.

97strokerHD · Apr 1, 2015

As long as the f super duty master cylinder will bolt up to my vacuum booster it should work. I could just use a proportioning valve from an f super duty but the extra volume would be nice for the larger amount of force required to run calipers instead of wheel cylinders.
Old Man Dave do you know if that master cylinder will bolt up to my vacuum booster because I don't want to switch to hydroboost.

kip4234 · Apr 1, 2015

5C3Z.2140.LB Will bolt right up. 1 3/8" bore

gnxtc2 · Apr 1, 2015

The F-SD master does not bolt up to the vacuum booster.

IMHO: I would swap over to the hydroboost.

Read these posts I made on PSN:

http://www.powerstrokenation.com/forums/showpost.php?p=1839290&postcount=8

http://www.powerstrokenation.com/forums/showthread.php?t=133798

Read the hydroboost thread, there is a ton of information in it. The last few pages speak about masters and proportioning valves. There is a guy that used vacuum/rear disc set up and the truck didn't stop correctly.

http://www.powerstrokearmy.com/forums/showthread.php?t=3940

Billy T.
[email protected]

m j · Apr 1, 2015

97strokerHD said:
As long as the f super duty master cylinder will bolt up to my vacuum booster it should work. I could just use a proportioning valve from an f super duty but the extra volume would be nice for the larger amount of force required to run calipers instead of wheel cylinders.
Old Man Dave do you know if that master cylinder will bolt up to my vacuum booster because I don't want to switch to hydroboost.

'extra volume' does exactly the opposite of 'extra force' in brakes
bigger master means less force, smaller master means more force

http://www.markwilliams.com/braketech.aspx

One of the most common misconceptions is that a larger master cylinder will create more pressure. While a larger master cylinder creates a larger displacement, it takes more force to create the same pressure as a smaller bore. While a larger master cylinder will take up system slack with less pedal stroke, it will take more force to create the same system pressure. The result after adding the larger master cylinder is a harder pedal which needs much more pedal pressure to create the same amount of braking force. For instance, moving from a 3/4" master cylinder to a 1" requires 77.7% more force on the push rod.

old man dave · Apr 2, 2015

Sounds like mj likes to post without reading what the thread is actually about. He does love to quote what he googles up, though.

The extra volume is needed since discs use a larger volume of fluid to operate than drum brakes. Poster did say he wants to install discs.

The pedal effort is addressed by changing the booster linkage attachment point on the brake pedal, restoring stock pedal effort or less, even with the larger master cylinder.

Its all about leverage, but you have to understand what leverage is. Some don't. They just quote alot.