I have a weird problem on my 2006 Road King Custom. I changed a tire recently, and ever since then my rear brakes go soft, if not all the way down to the pedal. I can pump them up and they will hold pressure and work normally... until I start the engine. Then they lose pressure and I have to pump them back up again. This bike does not have ABS, just standard brakes. I do not see anywhere that appears to be leaking, and there is plenty of fluid in the reservoir. I did not loosen the lines or do anything to introduce air into the mix. I just pried the brake pads apart a tad to get the caliper off. I have flushed about half a bottle of clean brake fluid through (fluid was still pretty clean) and bled them. Same results. I assume something is worn out, and when the engine starts, the vibration breaks the seal and pressure leaks out. Master cylinder need a rebuild? Has anyone run into this before?

 

If you see no fluid outside the braking system, there is either air in the system or the master cylinder is leaking internally.

 

Have you ever cleaned the pistons in the caliper? I can visualize grunge on the pistons catching on the bore seals and drawing the pistons back into the caliper when the brake pressure is released and to get braking again, the pistons must be pumped out farther than normal to get braking each time. It would be a coincidence, but not impossible that the master cylinder is failing at the same time the tire was changed. This is frequently caused by the longer first stroke to get the caliper pistons back into their normal position after being forced back into the caliper. The master cylinder piston is scraped against a part of the bore that has become dirty and pitted and the seal is damaged.

If you haven't done it before, there are instructions in the maintenance section of this site and on youtube on cleaning the caliper pistons. If that doesn't restore normal braking, a master cylinder overhaul or replacement would be next. One of the master cylinder failure modes is that the pedal will slowly go all the way down to the stop under light braking pressure but will hold pressure under heavy foot pressure. Try light, and then heavy pressure and see if the system responds differently to different pressures.

 

Concur with the above about cleaning the caliper.

 

You likely bent the rotor when the wheel was off for the tire change. You can pump up the brakes and they will hold tight until the bent rotor turns and pushes the pads apart slightly. Press the pedal and you can close the pads again but the rotation of the bent rotor will push them back out as you ride.You might be surprised how little it takes to cause this problem.

 

I found the only good way to bleed brakes on these bikes is with a vacuum. I picked up one at harbor freight. I did just do a master cylinder for a friend on a 02 but it was nasty, even if not dirty it does sound like yours is bad.

 

I 100 percent agree with dirty pistons.
I ran into the same thing when i did my front tire on my 04 EGC. I bled the heck out of the front brakes and it didn't fix it..

I finally rebuilt the calipers. I could not believe how much dirt was within the seals and pistons. Very easy to rebuild. Also I put on speed bleeders, easy.
Go check out the brake section forum here in "Tech and Mechanical". Perfect explanation of what to do.

Good luck

 

Which type of brake fluid does 06 have? My 01 has the silicone which does have to be flushed, my 07 has the caustic stuff, which is not as sensitive. But I don't know what the engine running would have to do with brakes.

Having to pump up, can also indicate not enough brake pad.

 

The thickness of the brake pads will have nothing to do with "pumping up" ... As mentioned earlier either there is a hydraulic problem ( air in the system or an internal bleed/by pass in the master ) or an mechanical issue with the pads such as the brake pads hanging/ sticking on the pins, pistons sticking or caliper not mounted correctly ... It is strange that the issue only arises when the scooter is running ... Just for grins and giggles have you raised the scooter and checked that (,possibly ) you've got a bad loose wheel bearing and the wheel is "cocking" and pushing the piston in? Stranger things have happened

 

Welcome to the forum.