I just reread your post about removing the rubber flapper valve from the booster connector....sad to say, but that is most likely the issue...
If you told the sales person the engine was an LS and you needed a booster for that engine, that valve MUST be in place....I can almost guarantee the booster connection is just a 90 degree fitting, with the booster check valve expected to be in the engine port....try it and see...
I will leave the rest of the original response for everyone general knowledge...
The standard brake pipe block on a drum/drum truck is not a proportioning valve....it is a simple trip over alert system to warn of a brake system pressure loss in either the front or rear brake circuits.
Often when bleeding a system the trip valve will fail over and block off one of the brake circuits....to correct this, remove the electrical switch from the block then pump the brake pedal several times.
A disc/ drum system must have a proportioning valve in the system to prevent excessive pressure generated by the brake system from locking up the rear brakes. The proportioning valve usually has a brake fail switch included...and again, often these drop into fail mode when someone is bleeding the system.
I always remove the switches before bleeding any system, bleed then once fully bled, replace the switch. I also prefer a pressure bleeder system over pumping pedal style of process.
A disc/drum master cylinder is a totally different item to a drum/drum master.
One of the most often overlooked items to check is the booster push rod depth into the master cylinder. It must be correct...too little depth into the cylinder and you will lose cylinder travel....too much depth into the master and the front disc system may not return correctly and lock up the front system.
Another often overlooked item is the pedal pivot position to the pushrod that goes into the cab side of the booster...an incorrect position of the pivot point will result in hard pedal or loss of travel....
Did you check the amount of protusion of the booster to master cylinder push rod?
Show us pics of both the back of the master cylinder and the front of the booster please.
Show us where the vacuum hose from the engine connects to the engine and then to the booster...close up pics please...
Have you taken a vacuum reading of the available vacuum from the engine at idle, from the booster hose fitting on the engine and again at the end of the hose where it meets to booster...
Thanks
