Although not operating 737s, we have had problems with hydraulic fluid becoming acidic by pyrolysis. With brakes set, any heat will transfer to the now stagnant fluid and, with carbon brakes in particular due to the higher operating temperatures, cause phosphoric acid derivative to be produced in the fluid. Heat also accelerates the effect of oxidation and hydrolysis. Both of which produce acids. After having made crews aware that the parking brake should be released once chocked, the problems have decreased considerably.