Help with Master Cyl Selection

Truckin222 · Aug 27, 2020

Hi all,
I have GM 3/4-one ton calipers (JB6) on all 4 corners of a heavy (5k) Jeep cab truck. D60s
Still have stock booster and master (88 J-10).
Pedal is soft and braking power is only ok.
System has been bled with vacuum and manually. I am reasonably certain there is no air present.

I'd like to find a master cyl that is more appropriately sized for these calipers, preferably 4 wheel disc.
I'd like to find a parts store (OEM reman?) solution if possible.
Best case something that would work with my existing booster.
Hi zoot aftermarket isn't in the budget just now(ever).

Any suggestions would be appreciated.

TBItoy · Aug 27, 2020

how close is a say 2005 2500HD Chevy master cylinder to bolting up to your stock booster?

(what kind of booster is it...?)

zukimaster · Aug 27, 2020

Man, I'm going to watch this thread, I have the same calipers, with dual 3/4 Wilwood master cylinders, I have bled at least a quart of fluid through each one, have no pedal! You can pump it up several times and it's ok, let it sit for 10 seconds, and it will go to floor, driving me nuts

Bebop · Aug 27, 2020

zukimaster said:
Man, I'm going to watch this thread, I have the same calipers, with dual 3/4 Wilwood master cylinders, I have bled at least a quart of fluid through each one, have no pedal! You can pump it up several times and it's ok, let it sit for 10 seconds, and it will go to floor, driving me nuts

get some residual valves. And maybe 7/8 masters

zukimaster · Aug 27, 2020

I have 2lb residual valve's on both master cylinders..,

Bebop · Aug 27, 2020

Well, that blows...
I know it's like shooting at the problem with a bazooka, but hydroboost and done.

zukimaster · Aug 27, 2020

I would, but absolutely don't have the room, I have the reverse mount master cylinders that are under the dash

Bebop · Aug 28, 2020

Truckin222 · Aug 28, 2020

So a look at Rock Auto shows the existing MC cross reference to tons of GM 80s trucks. C10-C30, Suburban, Jimmy etc.
RAYBESTOS MC39413
Booster was used in all FSJ in the 80s as well as Buick and Cadillacs in the 80s.
CARDONE 5471042

I know next to nothing about GM stuff, so not sure how close later model stuff is to bolting up?

I really don't want to go the hydroboost route.
My ram assist is already making enough heat as it is.

FlatlandFSJ · Aug 28, 2020

zukimaster said:
Man, I'm going to watch this thread, I have the same calipers, with dual 3/4 Wilwood master cylinders, I have bled at least a quart of fluid through each one, have no pedal! You can pump it up several times and it's ok, let it sit for 10 seconds, and it will go to floor, driving me nuts

How are the flex lines routed at the caliper/links? I had a similar issue while bleeding rays brakes the first time. The lines looped up and back down to the calipers, pumping the pedal and bleeding them resulted in a nice solid flow out the bleeder, but the pedal never would stiffen up afterwards. We started bleeding each fitting in the lines by loosening one at a time and stomping the pedal to the floor and tightening the fitting before letting up on the pedal. I'm not sure if it was the real cause of the issue, but when I got to the caliper lines and repeated the process at the fitting on the caliper there was a good bit of air stuck in the lines where it looped up and back down to the calipers.

I guess just pumping and cracking the bleeder wasn't allowing enough to push the air out the top of the flex lines.

Chaseweston · Aug 28, 2020

Can you get a dimension center to center of the booster studs? I can measure mine tonight. My situation is dealing with a newer vehicle (07 jeep jk) so idk how applicable it would be but I had spongey brakes under-powered using my stock master with my 1 ton dual piston calipers on all 4 wheels. I upgraded to a dodge ram master (Raybestos MC391385) looks like many guys do it on TJ's with one ton swaps and it bolted right up and stops on a dime now. This is the same master that comes with the big brake kit on the military J-8's, that is a poorly documented detail.

TBItoy · Aug 28, 2020

here's a raybestos catalog https://www.brakepartsinc.com/dam/jcr:51c828fd-2af7-447f-8036-7f09e65605e6/BPI-MCAG19A.pdf

looks like you have a 1-1/8" bore MC for the small brakes/half ton.

MC390278 should bolt up (and have the same 1/2" and 9/16" brakeline fittings), it has a 40mm bore (1.575"). should move plenty of fluid to firm up the pedal. It's the stock MC for a lot of 2500/3500 trucks, vans, etc with 4 wheel disc brakes.

Truckin222 · Aug 28, 2020

Thanks guys. That's the kind of info I'm looking for.
I'll track one of those down and try to verify fitment.

I'll measure the studs when I get home to make sure and add them to this thread.

Chaseweston · Aug 28, 2020

Truckin222 said:
Thanks guys. That's the kind of info I'm looking for.
I'll track one of those down and try to verify fitment.

I'll measure the studs when I get home to make sure and add them to this thread.

If Nick's predictions regarding your current master are correct, go with his recommendation on the larger. The one I mentioned is 1 1/8" bore and 7/16" primary, 3/8" secondary fittings.