Get a length of vacuum hose the same size as is on the parking brake release diaphragm. Pull the hose off of it and replace it with the length you begged, borrowed, stole or bought. Now put a hand held vacuum pump on the open end and pump down a vacuum. See if the gauge on it holds vacuum. If the vacuum holds, the diaphragm is good. If vacuum falls, the diaphragm is bad. If you don't have a pump (not very expensive), just suck on the hose and plug it with your tongue when you have draw all the vacuum you can. The vacuum will hold it to you tongue. If it leaks down it will fall off. If the diaphragm tests good, then work your way back towards the supply at the firewall.