Not sure about your timers. If it is just the Grasslin type that acts up buy some other brands. They cost under $10 in Target etc. and allow for minute precision.
I find it advantageous to inject fertilizers back-pressure-less, just open ended lines over the rim into the tank. That way you don't have to deal with clogging check valves and back pressure which might fill your fertilizer bottles with tank water.
I use peristaltic pumps for micros (Flourish) in a concentrated solution, and there are few clogging problems. For macros, I use a little water pump as I mentioned earlier, which is extremely reliable, and as a side benefits, agitates and mixes the solution each time during dosing.
I have the 6x irrigation timer/relay setup ready to assemble under my desk, but haven't had a quiet hour or two to sit there and solder it all together. I'll share the results once I have them.