[wslade2]

I also have front and rear engine driven pumps. Had a similar experience. Long story short, air was getting sucked into the system. If seal is leaking at either pump, it can suck air in, mix it with the fluid and foam it up. Foamy fluid compresses easily and makes actuation soft. Foamy fluid appears to “boil over” out vent tube. Beware possible leak on suction hoses and any fittings on the suction side. O rings Teflon backup rings also suspect. Also learned these pumps are not self priming. Be sure they’re primed while going through everything. Lastly discovered the backup of having twin pumps was no good because the check valves had 50 yrs of crud in them. Imagine a failed pump with check valve that won’t completely close allowing pressure to bleed off.

I would examine both pumps carefully. I created a vacuum device with hand brake bleeding vacuum pump to assess for suction leakage. But didn’t get to use because I discovered my front pump running dry after a run up where I had primed it. Ended up having both pumps rebuilt not simultaneously which lead to discovery of check valve problem. After pumps and suction side examination, do the check valves. One in rear wheel bay and other by power pack interior side of firewall as I recall.

For good measure I replaced all pump hoses, o rings and backup rings.