I'm going to risk getting in trouble with P. E. Haiges and say again change the pump. It's probably cheaper than you think. You say the pump was doing nothing but hissing when you pumped it, which really sounds like the pump itself is leaking. And that pump, which is the old style, is well documented to be a leaker.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References