I've pulled the hoses off of the nipples & back flushed it with a garden hose nozzle modulating the pressure somewhat with my other hand which helped with a slightly to moderately clogged one. No opinion on a good chemical to use but it'd have to be caustic to do any good. Cooling system deposits are tough. Doug Dutra the /6 guru used a solution of Muriatic acid (swimming pool cleaner) in a bare block & said the insides came out like fresh bare metal. Dont remember what percentage of acid/water he used nor how long he let it sit tho used in a core that might affect its' soldered joints