no pressure In rear brakes

[Red Herring]

The design of a caliper generally introduces fluid in the centre or towards the top of the caliper, and the bleed nipple is always at the top. This means if you don't drain them before flushing you invariably leave an amount of old fluid in the bottom half of the caliper, and given this is the very part of the brake system where the fluid is working at it's hardest this doesn't seem logical. This isn't such a problem if you replace the fluid regularly as you say you do as the new fluid will very rapidly absorb the excess water vapour from any old fluid still in the system and perform accordingly, it just means it will have a shorter life than if you had drained it completely.....

The same is not true of forks however. Fork oil is not hydroscopic like brake fluid. Water vapour will mix with it and form an emulsion (anyone seen how milky engine oil goes when a head gasket goes...?) and if you are in the habit of replacing the fluid in your forks by sucking it out from above you absolutely don't get the fluid from the bottom of the forks out (the design won't let you) and this is exactly where the contaminated fluid will be. I suppose if you go for a decent ride immediately before doing it you might be in with half a chance of getting a percentage of the crud as unlike brake systems forks do circulate the oil around them, but it doesn't take long for it to sink back to the bottom again where you will only remove it with a strip down.

If you doubt me get two small clear containers and put some brake fluid in one, and fork oil in the other. Now add a little (little!) water and give them a good shake. The brake fluid should remain fairly clear, maybe a shade or two darker but it will still be clear and if you leave it overnight it won't change. The fork oil will go all cloudy and in the morning you will see that it has started to separate again.

Changing either fluid regularly is good practice and as you say if you make it easy then it's more likely to get done, and if easy is good enough how can you argue with that.

[SV650rules]

I know bike owners that have never replaced either brake fluid or fork oil, and their bikes are over 10 years old ( just hope they replaced tyres ) - when a job seem too hard many people just ignore it - I am quite happy to do 80% of a job regularly because it is so easy rather than keep putting it back into the 'too hard' basket.

I never said fork oil was hygroscopic - it is petroleum based, brake fluid is a synthetic ester and brake fluid is designed to absorb the water and spread it around the system rather than letting it pool at lowest point and cause problems ( read about silicone fluid, it does not absorb water but the downside is it allows any water in system to gather in slave cylinders and cause problems ) - i just said leaving fork oil in too long causes a lot of the crud.

It is just my way, and I can say I have never really had problems - I never fully drain coolant in cars - I know people who have stripped threads or broken off the plugs in cylinder blocks while trying to get every last drop out, I just drain about the 50% or so that comes out of radiator tap, measure what comes out and top up with 50% premix ( I know there is a problem if same amount does not go back in), then run the car for a week and repeat - reckon I have replaced about 70 to 75% which is OK ( I fully drained a system once and it took forever to get airlocks out, swore I would never do it again ). Replacing coolant in bikes is pretty easy compared to cars so that gets done fully.

[R1ffR4ff]

A couple of times I've had problems getting pressure in Motorcycle brakes it's been a faulty Brake Master Cylinder. If the OP is sure there's no fault in the Caliper I would look to that.

[Toooldtodie]

Originally Posted by R1ffR4ff

A couple of times I've had problems getting pressure in Motorcycle brakes it's been a faulty Brake Master Cylinder. If the OP is sure there's no fault in the Caliper I would look to that.

That was what I was thinking. Mine went at 13yrs and the kit consists of some o-rings and a spring IIRC and retails for £45 or something stoopid. My rear seized on, but I assume the opposite could happen with fluid leaking past the o-rings?

[SV650rules]

Thing is when seals fail in slave cylinder the fluid leaks, but when master cylinder seals fail the fluid stays in the system but no pressure developed...

[aj76]

Hopefully you managed to get to the bottom of the problem but when you rebuilt the rear did you make sure to grease the bolt(5) and sleeve(10)?

https://postimg.cc/v4XYJfvy

Rather the the piston not moving enough could be there isn't enough lubrication for the caliper to move and both pads to make effective contact.

just a thought