Random drops in oil pressure when i press on the brakes

[Acoldtruckproject]

So i have a 72 c10 with a 350 engine auto trans with very random oil pressure fluctuations, mostly they happen right when i shift or when i decelerate to low speed from highway driving. There is an oil leak which seems to be from the main rear engine seal which could be the culperate, but i also had to replace a broken check valve in the power brake booster, so im wondering if theres a step i missed when i put the new valve on. Also my in gear rpm will fluctuate nearly 200rpm in total when sitting at stoplights and the like. Any ideas will help, thanks.
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