Hi there I bought an ST4s at the backend of last year and I have done a thorough clean and service on it. When I collected the bike, I felt that the brakes were dragging a bit and really quite hard on the lever. Never mind, I have replaced all the seals in the calipers. Job's a good'un. Went for MOT no problem. (it was in the wet) Today, I went out for a short ride, and it started OK, until the brake lever started to become harder and the bike felt like it was struggling. Sure enough, the pressure was building up and the front pads were solid against the discs. I pulled out. Figured what I had in the tool kit to relieve some pressure and eventually used the screw driver bit to pull the pads off the calipers. Enough for the bike to move and make my way back home on the rear brake. Now, I am researching the topic and it sounds like build up of pressure in the circuit because the return channel could be blocked. OK - I dismantle completely the brake master and to my shock, of the 2 ports, the rear one is fully drilled through but the front one (pressure relief) isn't. Is that right? Please let me know before I start getting the dremel out! And if it is not right, how would that bike have done 19,700 miles? The brake discs have been changed - I now assume because the older ones probably overheated and warped in the past. But the problem would have been obvious and stopped the previous owner from any rides I would assume. Thoughts from the experts? Should I drill that missing hole?
For reference of the functioning of a brake master cylinder Now that I think of it as well, the bleeding of the brakes was an absolute pain in the backside and I had to use some seringes to fill up the master through the bleed valve before I could get anywhere. Maybe this has to do with it as the air in the system would not come through the return valve and would have had to go through the seal,
You haven't left any play between the lever and the end of the piston. If there isn't a little free play the piston won't return fully home, keeping the vent back to the reservoir blocked and creating the symptoms you have been experiencing.
I can look into this, but it is not even close. The piston of the master is on a spring and it is constrained by the circlip. I am pretty sure there is some freeplay, but even if there wasn't, don't see that being the problem. The total length of the piston is about 30mm. The only hole that goes through is at about 20mm from the circlip and would therefore seat behind the front seal (exactly as per the diagram), whereas the hole that is not drilled through would indeed be just in front of the piston. So, my question is - Should that front port be drilled through and is it a machining error from Brembo? Therefore, should I take my dremel and finish the job? Or is it normal to be this way for a reason that I don't completely understand. Please clarify if you are in the know. The only other hypothesis I can imagine, would be that Brembo only have one port and that the piston should be shorter and somehow it was changed by a previous owner to the wrong specs.
If there is plenty of free play at the lever is it possible that you have re-assembled the master cylinder incorrectly? The bike is far from new and must have worked previously with no issues. I'm sure that you shouldn't be having to drill a hole.
That's exactly the problem - but see my problem - the compensator hole is not even blocked - it looks like it was never drilled through. So I can try and adjust what I can, but if the hole does not go through, I don't really see what the adjustment is going to do about it.
Looking more carefully there might be a pin hole, only that my pins are too big to go through those - that would explain how the bike did the mileage prior to the brakes grabbing. I have orders some printer cleaning pins of various sizes from 0.2 to 0.6 mm and will see if I can free the port with this. Brad says that he uses a 0.6. I will report if I do.
Is it a genuine lever fitted or aftermarket? I had this on my NSR250 and it was an aftermarket lever not allowing free play between it and the master cylinder. Nearly threw me over the bars going down a hill!
Genuine lever and I assume genuine pin. The port for compensation appears blocked / like it was never drilled. But I am reading the this is a tiny hole, not as big as the feed hole. So, I ordered some tiny pins to see if I can go through. Putting some WD40 in it at the top and the master at an angle so the compensation port is the only one in the WD40 - it is not dripping into the main cylinder. So, it must be blocked indeed.
While blocking of the vent channel is common, either with dirt or more often badly adjusted levers, if the bike had been fine for all that time, I’d go back over the work that’s been done and check everything is ok there first.
The previous owner says that it was not a problem, but I find it hard to believe him, as I noted that the brakes were dragging when I collected the bike with a wooden feeling on the master cylinder - very little travel on the lever. This prompted me to change all the seals of the callipers. And the brakes behaved for the time to go to the MOT (in the rain) and start another ride today. So, now that I have the ports exposed and getting some various size pins tomorrow (the smallest wire I have indoors is 0.85mm) I am going to try and see if I can free the port. then I will check that with the brake lever on, the front port is not obstructed by the front seal. As I am trying to figure out the story - There is some servicing from reputable places (JHP, Ducati John, GTEC) - so I can't imagine those places not addressing the problem. But that goes back about 4 years. I imagine that the problem developed and the seller probably damaged the original brakes discs. Changed them and then the bike essentially got ridden to go to MOT and back and eventually got SORNed. So the issue would probably have developed and it was easier to sell then get to the bottom of it. I'll try and free the port, or i have a 999 master spare. Might be a good idea to upgrades long as the clearance with the fairing works out.
