When my tandem pump failed, it did not make noise, it just didn't pump hydraulic fluid. I tried to bleed the system at the bleed screw on the control valve, and nothing came out. I then disconnected the high pressure hose from the tandem pump at the connection on the right side of the engine, and no fluid came out with the engine running at fast idle. Bad pump- I bought a rebuilt one from MBpumps.com in Houston TX.

Also, be sure the low pressure feed hose from the reservoir to the pump has not aged so much that is is blocked off. I would remove it and check.

"leak happened with the engine off, which is weird since there shouldn't have been any significant pressure in the system." Not true- the hydraulic fluid stays pressurized even when the engine is off, which is why the rear end doesn't sag everytime you turn off the car, and why you saw leaked fluid under your car forced out by that pressure.

"will drive it tomorrow and hope it bleeds itself"- haven't the other posts pointed out that that won't happen?

There are many many threads here about SLS, proper bleeding, etc. Use the search feature above.

The 87 is a great diesel wagon- good luck