Help with C4 brake boost problem please

[TRE Cup]

Hi
We have a 90 C4 in the shop. All of a sudden the brake fluid is leaking from the assist pump (mounted on the front trunk panel- looks like a starter motor with a green switch). Okay- must be a bad pump- right? We had a known good one and installed it. Less than 20 miles later and it is leaking.
Okay, time to be patient and see what's up. Install factory pressure guage, indicated accumulator problem. We replace it, install yet another pump, bleed the system and check pressures- all okay. 10 miles later, the 3rd pump is leaking!!
Abs brain is intact- no evidence of water damage- common problem on early C4s in wet climates.
Anyone have this problem before? Thanks!