Help! Brakes don't work after front caliper rebuilds.

[distantcamera]

My friend (who is a qualified mechanic and who has completely rebuilt two bikes from the frame up before) recommended that we rebuild the front calipers when we last changed the brake pads as the pistons were sticking.

We did that today. Used a rebuild kit for each caliper, polished the pistons and put it all back together. It didn't go so badly.

But the brakes don't work! I'm not sure if we're failing to bleed the brakes properly or if we have done something else wrong. We have bled through a full litre of brake fluid. There doesn't appear to be any air bubbles left.

But the brake lever is weird. If you pump it about 5 times the brakes start to work (but feel a bit spongy). However, let it go for a few seconds and the next pump the lever goes right back to the grip and you need to pump another 5 times to get braking. It's like the pistons are actually retracting and need to be pumped out again. Could this be caused by some illusive air in the system? Or have we screwed up somewhere?

Any ideas gratefully received. I'm stuck in a wee town with no motorbike shop and I need the bike to take me home for work on Monday!

Thanks in advance.

[TLPower]

Clamp off the hose as it comes out of the cylinder and pull the lever. If the lever still comes back to the bars the air is in the cylinder, if it doesn't the air is in the calipers.

It is air in the system, open the bleed nipple (one side at a time) after each "pumping" (5-10 pumps) this should clear the calipers. Close the nipple before releasing the lever. If that doesn't work try a syringe on the bleed nipple to "pull" the fluid through.

Did you let the master cylinder run dry? If so repeat the 5-10 pumps and slacken the brake pipe fitting on the cylinder. Tighten the fitting before releasing the lever. Have plenty of water on hand if you spill any fluid onto paintwork, if you do spill any, pour the water on to the fluid, don't be tempted to wipe it.

Finally tie the lever back to the twist grip and leave for a couple of hours, this should clear any remaining air in the cylinder.

Sorry if I've gone on a little and please apologise to your mate if I'm teaching him how to suck eggs.

[hookie]

It also helps if you gently tighten the bleed nipple after you've squeezed the brake lever each time. This stops air being sucked back into the caliper and helps oil get drawn from the master cylinder into the hoses.

[Gert]

What model is your bike? Does it have ABS? It sounds like you may still have air in the brake system. ABS systems are a little more stubborn to bleed.
Best to have two people when bleeding the brakes. One to pump and hold pressure on the lever, the other to open the nipple and close the nipple before the 2nd person restarts the pump and hold process again. Remember to bleed one caliper side to purge any air before starting the same process on the 2nd caliper.
Another alternative is to use a syringe to force brake fluid in from the nipple side to fill the master cylinder..

[oslin]

Good advice there, but an alternative would be to use a vacuum unit they have worked well for me but you must remember to keep the master cylinder topped up during the operation.
if you have an ABS unit fitted it can be a bit of a pain getting the air out of that, I always avoid draining that unit by just pumping new fluid through it.
good luck with it all.
Keith

[sjrainsford]

One more to add on the ABS front.  It also helps to activate the ABS pump to force any trapped air through it.  Often it's the only way on an ABS system.  Without diagnostics kit you can sometimes get it to operate by spinning the rear wheel for instance while you apply braking force to the front, this will activate the ABS pump due to wheel speed difference.  However, you first need to ride the bike a little to activate the ABS system (until the light goes out).

[distantcamera]

Thanks folks! We shall try all of the tricks suggested until the beast behaves. I'm surprised it misbehaves so much. My Honda Deauville took about 5 mins to fill and bleed after removing the calipers and master cylinder and having the most of the system empty.

[Joe Rocket]

Yep, I did two front and one rear caliper rebuild then bled them, along with hydraulic clutch on my Kawasaki project........ as per TLP above. Needed lots of fluid, I bought a new bottle but used up old tin for cleaning and initial flush right through. Have to pump fairly quickly to take the air bubbles with the flow of brake fluid and expell them out of the nipples.

Front pair more difficult as you have to go back to the other one just to make sure a couple of times. If you've let any air back in via the nipple(s) you have to start again. 

[distantcamera]

Hmmm. I think the bike is going home on a recovery truck 

[distantcamera]

It did go home on a recovery truck. Now parked in the garage with a cable tie on the brake lever.

[The Border Riever]

If you have access to a compressor with a decent sized reciever, one of these will end all problems with bleeding brakes on any vehicle........https://www.ebay.co.uk/p/Sealey-Vs020-Brake-and-Clutch-Bleeder-Vacuum-Type-1ltr/12031170110?iid=392003648801&chn=ps