It’s hard to believe that ABS brakes are offered for that vehicle but they are and possibly that could be causing the problem.
There’s a ‘sky hook’ out there to calm the nerves of the incompetent in all manner of situations except when the hook fails.
I don’t believe it has ABS, not sure though, I’ll check. On this
particular vehicle it seems to be void of many so-called safety
devices which I find very comforting. Exploding air bags may save
lives - or take them . I use this utility vehicle for work, even
with a “crane” to assist with lifting fairly heavy objects . I find
I can actually start it from outside the vehicle in gear if I wish -
sounds unsafe I know but hasn’t been the problem you might think it
is. Operating machinery takes “present moment thinking” and I
believe the more we get away from that concept the more unintended
consequences will occur.
Not sure where ’ thinking ’ is involved here .You start vehicle in gear and you are not in it - vehicle starts moving one can only hope it hits a solid object and not a child .
It is a proportioning Valve, not professionalizing!!!
I wouldn’t rely much on a mechanic at a normal Brake Shop.
That’s like taking your Rolex to the counter at Target for a new band.
Go find a specialty shop. Ask around at the events and find who everyone else goes to for this specialty work.
Yosemite
I suppose you’re directing this condescending post at me. Never endo’d a jeep before but watched plenty of them happen and been in situations close to it due to the extreme angles climbing various trails and features. The purpose was to impress upon the op that his vintage Jeep should not roll backwards under much less severe condition if we can stop same type Jeep from doing it. If your idea of my reaction to it is incompetence, would love to subject you to similar situation so you can display your superior competence…
I thought that might get your attention. Just saying the safety
regulations and many well intended devices and rules sometimes cause
unforeseen problems - think air bags killing babies etc.
Opps sorry to start a war of words - Thanks to all with helpful
advice. I was just a bit distracted by the spelling lessons in
stead of someone having a similar problem actually knowing the fix.
I'll be unsubscribing now thanks .No really - i need a professionalizing valve on this site to get
professional advice… of coarse I clicked on spellcheck’s suggestion
without my glasses. Hope you had as much fun as I did. Thanks to
all the actual helpful comments. Bye
No @TwinTurbo, just voicing my aversion to all the electronic snake oil used to sell cars. I built an M38A1 from the ground up from 3 scrap yard models. My boys wanted 3 wheelers but those just didn’t suit me. With a roll bar and seat belts the old Jeep always seemed to be a big thrill for the kids and everyone got home in one piece even if it required tipping it back over on the wheels occasionally. I do recall disconnecting anti lock brakes on tandem axle trailers long ago. They were a problem from day one. And for off road use most of the modern marvels are added problems instead of solutions.
the early 90’s were the golden age of modern cars imo, at least for diy’ers. Computerized electronic fuel injection, clean burners, but good chance there’s no air bags, no abs, no complicated evap systems, etc. hope you are able to fix the brake problem and keep 'er going strong. I’m daily-driving a Corolla of similar vintage.
Just pointing out the proper term. Unless you don’t mind all the guys at the parts store laughing at you for asking for a professionalizing valve.
If you are that touchy…good riddens. People were trying to help.
Yosemite
Is that anything like good riddance? 
No. It’s like a proffessionalizing valve.
And OP, inspect and adjust the brakes.
Wonder what type of proper terms we use when it comes to engine designers when you have to remove 2 bolts to remove an alternator and have to disassemble 6 other things to remove it from under the hood. Please don’t post examples.
Job security!!!
Yosemite
Praise the designers that they didn’t force you to remove the hood? 
All of this really may boil down to needing to install a “Big Wheel” style handle brake to lock up the rear brakes as needed in 4 wheeling situations. In normal operation the rear drum brake is not the primary stopper…the front wheels are. The rears are involved but they don’t hold like the front will or how you may need in your extreme situation.
If your rear drums are not adjusted properly it adds a lot of pedal travel to the system because that space needs to be taken up in the rear drums…it can really mess up stopping ability. Again…in the situation you describe you should be getting rear wheel braking accomplished, maybe it just isnt enough for the situation.
Aside from the rear drums being adjusted properly another often overlooked item is your rubber flex line to the rear axle / brakes. They fatigue and blow up like a balloon upon braking, robbing your brakes of power and increasing pedal travel needed (or pumps) for lockup. A rear Stainless braided line or a new rubber one can solve a lot of brake sponginess / power problems.
I’d be looking at doing a disc brake conversion.
That wouldn’t hurt … and if he did do the conversion… a new rear brake line would be involved most likely.
Many people never realize or suspect that their rubber brake lines can go bad, not by leaking but by fatigue. They literally swell up upon braking its not a good thing. This is why stainless braided lines exist…they can change braking power and characteristics massively.
I worked on a modified CJ-5 long ago that had the parking brake operated with a European style lever between the seat. The brake cables were in a left/right/both selective bracket and pulling the handle didn’t lock the brakes unless a lever was squeezed. I had seen those brake mechanisms advertised for dune buggies long ago and thought they might be helpful off road. Just moving the hand brake to a lever between the seats would seem to be a big help when driving off road.
There’s still a lot of peculiar hand brake hardware available for dune buggies that no doubt can be used on any vehicle