When I had to replace a brake line on the 190 I could not get it to bleed until I got the pressure up to 15-20 lbs.
The one I made is a pump garden sprayer that I added a pressure gauge to. I bought a new reservoir cover and drilled the old one, which fits plenty tight. I loose pressure out of the O ring around the pressure gauge right at about 20 psi.

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95 E300D gave away
77 300D, 227k, station car
83 300CD 370k, body gone away to the rust gods, engine is in a Yota pickup, going strong
89 190E 2.6- 335k, no more
79 VW FI Bus- 154k summer driver
59 VW Beetle ragtop- 175k
12 VW Jetta- 250k
74 MG Midget-78k