Brake Pedal Going to Floor in my 87 2.5L

[MississippiComanche]

Checked vacuum, it's good. Also replaced check valve. Fairly confident my brake booster has a torn diaphragm. I get pressure when truck is off but it goes to floor and hisses when it's cranked.

 

I guess now is a better time than ever to do an upgrade to the booster. I've never replaced booster on any of my vehicles and know little about the upgrades, so any advice is greatly appreciated. 

 

Also I have an old rusty master cylinder. Should I replace it too or keep using the original? 

[eaglescout526]

How rusty are we talking? Course if doing the booster upgrade, I think youd have to chance the master too.

 

That being said, hows the pads and shoes? I had this issue where my pedal as almost to the floor due to missing the alignment pins and causing uneven wear on the drivers side brake pads. Replaced the pins and the pedal would almost be to the floor.

[schardein]

Normally I would say that a brake pedal that goes to the floor, with no obvious brake fluid leaks anywhere, would indicate a bad master cylinder.  Fluid is bypassing worn o-rings inside the MC.

But, if you are getting a hissing sound, that is an indicator for a leaking brake booster, as you've said.  If you have a Mityvac (hand pump that produces a vacuum for testing things and bleeding hydraulic systems like brakes), you could put a vacuum on the booster and see if it builds and holds vacuum.

Booster upgrades are well documented here in the Club.  Broad answer is 95-96 XJs and I think WJs?  Either booster upgrade requires a different master as Daeson said.

Another option if you are in a hurry might be to post a wanted ad for a stock booster.  I'd bet that some members have a stock one laying around after doing the upgrade.  It would be a quicker/easier repair than finding an upgrade booster if you don't have any junkyards near you.