lister explained :
To save (perhaps) a lot of aggravation, no.
When I change the fluid I do it by opening a bleed nipple, pumping the brakes to pump the fluid almost to the bottom of the reservoir, then top up with clean. Then I carry on by bleeding at each wheel until it runs out clean - making sure the reservoir is kept topped up. It just saves any air getting into the system
shazzbat used his keyboard to write :
We do all have to start somewhere, at least he has had the good sense to ask for advise first.
When he comes to bleed them and check for sponginess/ possible air in the system, I have another tip I learned long ago....
Check it without the engine running/ no servo working. There should be some movement down on the pedal, then it should stop with a very solid feel under your foot - no movement after taking up the initial slack in the system. If there is movement, you still have air to bleed out.
With the engine running, it will feel a little spongy anyway and it is easily mistaken for air - says he who spent several hours bleeding the system with no air to be bled on his first servo equipped car.
Can someone tell me why all the guides say to pump the brake pedal three times when bleeding?
Surely if you pump the pedal, then air will be drawn back up into the system on the first release of the foot? Or am I misunderstanding the operation of the brake system?
I would have thought you should depress the brake, release the bleed nipple, close the bleed nipple again to prevent air intake, then release the brake. Repeat as needed?
Am I misunderstanding something fundamental?
None of the ones I've ever read specifically mention three times; just pump until there are no air bubbles.
The construction of a normal car master cylinder gives it the effective function of a non-return valve, so the air will continue to move towards the open bleed nipple.
Most motorcycle master cylinders are of much simpler construction, and do indeed just move the air bubbles up and down the lines, so your description of a bleeding system is more applicable to them. They are often much more difficult to bleed than car systems, and it is sometimes necessary to use vacuum on the bleed nipple. (I've done this successfully in the past using a big hypodermic syringe.)
Some early clutch hydraulic systems also have simple master cylinders, and need to be bled in the way you describe.
There shouldn't be any means for additional air to enter the system; the bleed nipple should have a tube attached that ends in i container of brake fluid.
It would seem so, but I trust you now have a better understanding?
Chris
lister submitted this idea :
I have never seen that.
Don't pump it, or it can draw air back in.
Correct way is to slowly press pedal to the floor with the nipple open, close the nipple and allow the pedal to return. In practise you can get away with putting a finger tip over the outlet of the nipple to act as a one way valve, rather than opening and closing it. Requires two people or perhaps an Easi-bleed type gadget.
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