i had to take my aux water pump out as well, since it was shot. when i took it apart, it had more than a few burned windings. so im gonna have to pick up another one as well. even without the aux pump, i still get blistering heat inside. the reason i say this, is because you have a valve after the aux pump in your drawing. with the valve off directly after the aux pump. if the pump isnt being shut off when the valve is switched, then you could have some pressure building up and a leak develop. you might want to set it up where some flow recirculates back around to your other bypass hose when the valve is closed.

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1980 300D