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I just installed new brake lines / calipers and would like to pressure test the system before putting the body on as I would like to take care of any issues while everything is easy to access. Of course without the body I don't have the master cylinder installed so I need another method of pressurizing the system.
FWIW: at the factory, the master cylinder was installed before body drop (same as your situation) and the system was pressure fed and bled in one operation. See below:
You will have a REALLY hard time making a fixture that will allow you to actually actuate the master cylinder.
Pressure bleeding the brake system is one thing...and the pressure values are really low (20 psi+/-)...which may not show a leak...but when the pressure increase when the master cylinder is actuated...it can go easily up to 800+psi.
I do this often...the one connection I an super aware of is the rear line that attaches to the left side junction block. I have made a special crowfoot flare wrench that fits on my extension and ratchet that allows me to get into this area and tighten the fitting if needed.
I wanted to do the same thing. Ended up using air pressure. I know it is not as high as the brakes would see but it worked pretty well. I installed a tee on the line with a gauge in one port and a schrader valve on the other. Then put the pressure as high as I could get, let it stand over night and made sure the pressure was close to what is was when I had aired it. Obviously allowing for a slight drop due to the air cooling.
I wanted to do the same thing. Ended up using air pressure. I know it is not as high as the brakes would see but it worked pretty well. I installed a tee on the line with a gauge in one port and a schrader valve on the other. Then put the pressure as high as I could get, let it stand over night and made sure the pressure was close to what is was when I had aired it. Obviously allowing for a slight drop due to the air cooling.