The whole issue with the Jaymar style valve is it has an internal release spring - when you step on the brakes and exceed the pressure on the output side of the valve it will release itself . Also , if it loses more than about 10% of the pressure it's being locked at it will also release . I routed the lines on the Sami to a spot just ahead of the seat and installed two of them - all 4 were locked and it only sat there for less than 2 minutes before it took off with zero load on the brake system . That truck was possessed anyway , I swear at times it was trying to get even with me for treating it the way I did and using it as a dual-purpose daily driver to boot . After switching to the 2 stainless steel ball valves (rated for 5500psi) it never had that issue again and was regularly left overnight with all 4 wheels locked up .
Now, if anyone gets the bright idea to try this - be warned as there are some oddities to be aware of ....
First , using ball valves means the system is locked shut when the valves are closed , totally without question . If , for some reason you have either of those valves closed and try to drive it - no brakes . I mean , the brake pedal will not move at all , like the piston is stuck in the master cylinder won't move . I don't care if a gorilla stomps on that pedal - the master cylinder and booster is not capable of pushing fluid past a closed ball valve , period . So , if it sits and the system bleeds down for some reason (like a slowly leaking wheel cylinder/caliper) and you take off - no brakes . I had it happen once when I left the rears locked , it leaked down over night and I went to back out of the driveway at 4am - hitting the huge maple tree that got in the way . Just a bit too tired to do the pre-drive check of those valves - I had the handles oriented so they stuck out towards the pedals , sort of in the way of the foot well purposely to get attention when they were in the closed position ..
The benefit was I could load the brakes very hard and lock both those valves , holding all 4 wheels locked tight . It would pretty easily hold the truck in place when winching without having to hold the brake pedal while trying to hold the engine it higher revs to provide the needed amperage for the winch . It also meant the thing was basically pretty dangerous for anyone but myself to drive - almost no one could ever guess what the heck those heavy duty looking valves were there for - I always told them it was for dual fuel tanks , lol .
In short , it's really no recommended . Best system to use is a Mico lock in either the electric or one of their cable operated setups - they are very well engineered and designed for commercial trucks . The engineering company I worked for years ago had them on their large bucket trucks for doing aerial cable/phone work on poles and they worked great , as long as the rest of the brake system is properly maintained .
Brake Locks | MICO, Inc.
The cable brake lock mentioned as a hill holder on the old Chevy was probably one of the aftermarket kits made back in the day - I saw a few of them used by farmers when I worked in the diesel shop years ago . One of the old kits just used a simple cable to actuate a trailer tongue-actuated hydraulic lock-out valve designed to allow backing of a tongue brake system up a hill , just reversed . Not a bad idea but very few of those setups had a separate lock-out valve . Mico has a cable actuated setup that is very well built - some heavy duty trucks back in the day had no park provision in the Allison automatics - the park function was one of Mico's cable systems mounted on the floor next to the shifter .
Hope this helps - if anyone wants to build something like this I'd stick to a commercially-approved setup to avoid any liability issues...lol .
Sarge