jb1 manual master on jb3 booster

[87chevy.com]

With the current brake setup I have on it now, I'd like a little softer pedal. softer pedal means more travel.
I made an elaborate excel spreadsheet with the correct formulas and with the 2019 2 piston calipers 2" pistons, and the c5 rear brakes at 1.77", pedal ratio 4.6 means stock JB3 master 1.57" front brake bore, 1.125" rear brake bore is giving me total leverage of

Front: 14.9:1
Rear: 9.2:1

manual master cylinder on the other hand, JB1 is about 1/8 smaller bore front and rear, Front 1.47" and rear 1.0"

increase leverage to
Front 18.3:1
Rear 14:4:1

installed the summit 4 wheel master cylinder last year (and 4 months, out of warranty) on the stock JB3 master, seemed fine, no lock up problems, just firm pedal.

Booster tested and operates properly
The L83 gen5 vacuum pump puts out about 23-25 in of vacuum which is plenty

I had the plastic summit bleeder tool installed, mighty vac vacuum tool on the front caliper and I applied the brakes all the way to the floor slowly, loud pop sound... I stopped, inspected and found fluid started leaking from the sensor port. Took the valve out and apart and valve was no longer centered (obviously) and the tip of the tool was in side.

Re-centered it, no longer have a functioning tool, however I have one at work that I made from an old switch but decided to just vacuum bleed it only as its Sunday. Not sure if I didn't get all the air out by vacuum bleeding it only, but same loud pop was heard and no rear brakes (testing on jack stands. fronts locked/ cant rotate by hand. I will bring a borescope from work and inspect to see if its no longer centered as I'd rather not take the thing out unless I'm replacing it. But if i replace it, I don't want the same thing to happen... just trying to figure out what the problem is and why it sheared the tool off in the first place.. are you not supposed to apply the pedal all the way to the floor with a vehicle that has a proportion valve? in the past, with 2 person bleeder , one pumps and holds till it goes the the floor when the 2nd person opens the valve....

The ratio of pedal leverage is about the same front and rear
Front + 2.6:1
Rear + 3:1

Long post short, anyone else install a JB1 master on a JB3 booster.

JB5 booster from what I briefly read up on is a dual diaphram booster, but is larger and will not clear the valve cover trim.. so not an option I'll consider. Hydroboost? Don't have a vacuum problem and not sure how much additional pedal assist it gives over a single diaphram JB3 booster to make it worth trying.

thanks and hopefully someone has some input, if not swaping a stock JB1 master, but another aftermarket one with similar bore size... did you do swap the prop valve to something else?

clint

[87chevy.com]

forgot to mention I measured the JB1 piston to flange and on the one I have, matches the JB3 as well as piston to the push rod dimple...

also the switch on the summit 4 wheel disc does not work when its tripped lol... more like grrrr.... pulled the valve off again since i know the loud clack under the hood when braking hard is the valve shooting over to one side... yep its tripped, continuity from the single terminal to the brass housing shows no continuity, thus no light on the dash.... useless... removed the switch, pushed the pluger in and it makes contact with the teriminal... not sure if the rubber orings are insulating it or its just a crap design and needs a 2 pin switch.... going to call summit tomorrow and inquire about that

clint

[87chevy.com]

After modding an old master cylinder cap for pressure bleeding, I only dared to go 5 psi with the motive pressure bleeder I use at work due to the cap flexing and sketchy level 9. Reduced the sketchy level down a to 8 with a bar and a length of little rope chain like the universal ones need.... I was able to get the air out and she stops pretty dang good!

Defiantly an improvement over the stock power brake master cylinder. Doesn't seem like the pedal has more travel to me, but I didn't drive it for about a month and I don't drive it as much as I'd like... but in the end, it feels normal.
Clint

[SCOTI]

I've been watching & following this....

There are definitely multiple combinations that can be used to render the desired 'feel'. Trial & error is sometimes our only path. Glad it worked out.

[Keith Seymore]

Sorry to be joining late, but for future reference here is the brake release chart for the C10 product line.

This shows all the dimensions and part numbers for the entire brake system, and is the handy reference the engineer used in order to release the parts for production and service.

I grabbed an '85 because I already had it scanned in; '87 would be similar.

K