Forgive a bit of a numpty question but am wondering what the consequences / symptoms of overfilling a rake reservoir would be

Would this cause a dragging brake?

More likely to cause big mess but I suppose it could cause drag when the brakes heated up.

If the fluid level in the reservoir is high enough that the fluid is placed under pressure when the cap is screwed on, it could limit the fluid return on brake release. Can you not simply remove fluid until reaching the the max/min level?

The reservoir cap is vented so if the fluid heated up and expanded some would get pushed out through the vent. A syringe or bit of rag to soak some up should get the level back to where it ought to be

My experience, the cap is not vented but covered by a rubber diaphragm that expands and contracts as the fluid level changes. IMHO an over-filled reservoir may not leave room for the diaphragm to contract with a rise in fluid level when the brakes are released, effectively pressurizing the reservoir.

magg and newtothis are both correct. The cap is vented, and there is a rubber diaphragm. The diaphragm keeps the fluid from leaking out of the reservoir, and the vented cap allows the diaphragm to expand and contract within the reservoir without being affected by pressure/vacuum.

That being said, if you overfill the reservoir with fluid to the point that there is no room for it to expand when heated, you COULD cause the brake to lock up. As you use the brakes the fluid is heated at the brake caliper. This heat causes the fluid to expand, which if it has nowhere to expand to, will cause increased pressure on the pads, which will cause more heat and expansion until the brakes lock up.

That max mark on the reservoir is there for a reason, and in this case, more is not better.

OK so here's an update

Forgive the length of text and if you're not interested fair enough - I'm giving the detail because it's all a bit odd and I'm thoroughly confused - although please with the result

So the original problem was after a gentle 5 minute shakedown from a 10 year dormancy and two year rebuild, the back brake seized on so hard I couldn't push the bike. Even undoing the caliper mounting bolts and gently whacking the caliper with a rubber mallet, I couldn't take the brake off the disc. Although I did notice it was rotating about the axis of one of the pistons. I only managed to get it off by releasing pressure at the bleed nipple

So I stripped the caliper again and fitted new pistons from David Silver (one of the pistons did have a small pit - see image below), along with the already new seals . Doing this meant I had to bleed the brake and set the fluid level again.



Only this defect on the piston and the and the potential of over filling the reservoir could explain to me the cause.

So today I took the bike for it's MOT and new tyres which is a 20 minute ride in town. Right towards the end, two minutes from the shop the rear wheel started to drag again. I stopped at the shop and again I could barely push it. A mechanic greeted me and I explained about the brake. He propped it up on the stand, tried to spin the back wheel but couldn't and confirmed the brake had seized. He reckoned it was because "someone had put shiny new paint on the caliper". Well I did that and the bores were meticulously clean...



The garage told me they couldn't strip the brake and might not even be able to drain the reservoir but did agree to test for an MOT anyway. About 20 minutes in waiting to get new tyres fitted I see one of the mechanics easily moving the bike to the tyre bay - maybe the brake had cooled off I thought.

A few minutes later, a mechanic calls me from the waiting room to explain that the seized wheel was now spinning freely. Apparently, they had to slacken off the hub pinch bolt because the chain was tight - although he did say it was in reasonable condition. I asked him to pump the brake to test it still wasn't the problem.

So it sounds like I had two, perhaps three different problems causing the same issue. A pitted piston, over filled reservoir and a chain so tight it dragged the wheel enough to need plenty of revs to pull away. I just can't believe a chain would do that. Whatever, I rode the bike home, using the back brake as normal and everything was fine... I'm going back on Monday to get a new chain fitted

To the point about fluid levels being there for a reason. Absolutely fair point and I accept that. Although I am pretty meticulous. One thing I did find tricky was filling the reservoir to the upper level only to find that when the diaphragm was placed on top it squished into the fluid and it all looked over full. Second time round I veered toward the lower level.

BTW, if you're in London, I'd thoroughly recommend Watling Tyres in Catford 020 8936 3402. Very friendly, and seem to go the extra mile. The foreman understandably didn't want to strip the brake as I arrived at 1pm and they close at 3:30 on a Saturday. Although a bit annoyingly they couldn't fit the new chain I had brought as they didn't have a c-Spanner nor a drift to adjust the chain and didn't want to use a screw driver.

Fingers crossed...