Disc Brake Pad Gap

My first time with discs too so take this with a pinch of salt!

Sometimes it's not the caliper or the rotor or the pads.  My Malt G2X fork is on its way back to Merlin because, after me giving up trying to get my front disc brake to run without contact, the workshop I took it to advised me that the fork was out of alignment and the brakes would never be right using it.

The pad ajustment spacers are effectively a  feeler guage to centre the caliper as the pistons are operated....useful for setting up but if your pads are not retracting enough and "rubbing"...I'd suggest a bleed. Many people bemoan mechanical disc calipers but to be honest we get far less bother with the dual piston ones in the workshop than Hydraulics. Obvs the usual stuff is squeal... feel etc which can usually be sorted with new pads/decontaminate... your set up sounds like a set up issue for centring/bleeding out... can't tell 100% unless seeing it.... good luck getting it sorted

I think the tools are useful for centering, but the gap will return to what it currently is. After all you pushed the pistons back by hand, and then after applying the brakes the gao was back to where it had been before, so why would it be different with a saocer tool?

There's not usually any gap adjustment on disc brakes per se. One thing I've tried when fitting a new disc, is to undo the top bleed screw at the lever and then push the caliper pistons back in with the pads still in place. When the gap is suitable, replace the top bleed screw again - the theory is that there's space for a bit of fluid to escape if there's too much in the system.

Otherwise, you can try truing your discs so that they're within 0.5mm of true.

As a heavy rider (90kg) living next to the Jura in Switzerland, I get a lot of brake wear and piston issues. This is what I have found works for me - all my brakes are shimano, maybe sram are different -

1, even if you don't wash your bike it is worth running cold water through your brakes to rinse out dust. You could also use brake cleaner spray, but water is less horrible for the environment.

2, whenever I wash my bike, I take the pads out first (worth mentioning a tiny dab of copaslip on the thread of the pad retaining pins to stop it from sticking). Then I use a toothbrush to wash around the shoulders of the pistons. If they are sticky or very dirty, I use a spacer made of a few old links of chain in between the pistons and pump the brake level to make the pistons move further, make sure you use a spacer as if they go too far you are in trouble. Then I use thick cotton string like floss round the base of the pistons. Then push the pistons back in, a special tool or a plastic tyre lever avoids damage to the pistons. If you keep some old pads handy you can also use them to protect the pistons while you push them completely in. If you sense any stickiness, pump them out and push them back again. the two pistons should move the same amount on each lever pump.

3, Put the pads back in, wheels in, then pump the levers until the brakes work and you have good lever feel.

4, spin the wheel and listen, any scuffs reveal the rotor is out of true, so use a special tool or a clean adjustable spanner to bend the point of the rotor which scuffs. You only need to apply a tiny amount of force, better to make lots of tiny adjustments than risk taking it too far and having zig zag rotors. You can normally see which way you need to go by shining a light between the pads and looking for the missing gap. 

If I have a really sticky piston, I have found lubricating it carefully with the oil shimano use as brake fluid can help clear the crud out. But you need to be really careful to avoid leaving oil which could contaminate the pads or disks. 

Starting point is that if they are too close, they won't move out by themselves. You would normally prise them apart with a spacer tool or flat bladed screwdriver (trying not to score the pads, hence the tool) the idea being that this pushes fluid back into the reservoir.

If the brakes have been over-filled, then you can't always get the pads to retract. In that case I'd expect very little play in the levers.

Not messed with GRX levers, so not sure on what tricks, but as a general observation some systems are tricky to get set up with the right amount of fluid - too little they can be spongy, too much, they don't release.

Worth looking up the Shimano dealer manual which is publicly available.

Spacer tool would work but assuming you haven't got issues with sticky pistons/seals and don't have rotors that are out of true too much

Sounds like your pistons aren't perhaps retracting enough? How long have you been running them? Could the seals around the pistons need a squirt of silicon spray so they can slide back a bit easier?