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Front brake loses pressure

  • Thread starter Thread starter SeanGSUK
  • Start date Start date
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SeanGSUK

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Hello, I have a GS550. It came with front brakes that fade, and need pumping each time I want to use them. I have changed the hoses to braided steel and changed the master cylinder for a brand new one, but the problem remains.
I have completely bled the system twice and put brand new DOT 4 oil in it, makes no difference. I cannot see any fluid leak from the calipers themselves. Anyone have any ideas? Starting to be a bit scary!
 
with new steel hoses, and a new m/c and no visible leaks then it still sounds like air in the system. those steel brake lines can be a right PITA to bleed through properly sometimes.
pump the brakes till the are hard then cable tie the lever tight to the bars and leave over night. this can help to force any air bubbles up and out into the m/c
 
Thanks guys, I will try the cable tie on it, a friend who has a 70s Z says these old bikes are a PITA to bleed properly. Hopefully your advice will bear fruit, I will let you know...
 
Put a tie on there now, pads are ok, thanks, will check pressure tomorrow.
 
Hope you have better luck than I've been having. I've bled bottles of synthetic fluid through the system since March. Tied the lever back at least a dozen times. Seems to firm up, but ten minutes of riding and I'm back to a soft brake lever again.

Looking at purchasing a used GS500 MC off Ebay.
 
If you still have rubber hoses get some of these:
http://www.harborfreight.com/3-piece-line-clamp-set-97578.html
I think Sears sells them and maybe big box auto part stores.

Use it to crimp the hose near the MC if the lever is hard then the MC is good. You can also crimp off each caliper and see if one is causing problems.

It was very hard to crimp the 30 year old hoses. My new hoses are soft and crimp easy.
 
The brake is definitely firmer after having the lever tied back overnight. Still not 100% happy with it though. It already has braided steel lines on it. I am ordering new pads too. Hopefully they will help even though there is plenty left on them.
 
Air will give you a spongy lever, but if you mean by fade that the lever continues to move slowly while you are applying the lever (like there is a slow leak) then the M/C is bypassing its seals and you need to rebuild the master.
 
Another thing is to make sure the sliding pins in the caliper care in good condition and properly greased with high temperature caliper grease.

The tying the lever back trick is mostly to allow the caliper pistons to creep out of their bore closer to the disc, thus each lever pull won't have to push the piston as far (firming up the lever). Of course, if the caliper is full of crud all bets are off until you take it apart and clean everything properly.

Helped a member here a while back struggle with this same problem, only more extreme, and in the end the problem was traced to binding caliper pistons because aftermarket caliper piston seals were used which were too tight on the pistons. Big pain in the rump that one. If you need parts always make sure to use original Suzuki stuff.

For that master cylinder you used, if the piston diameter is less than 14 mm then you will get a spongy hand lever and no way to improve the situation without changing the master.

Good luck
 
Two other things that might affect your system:

1. You made no mention of rebuilding the calipers. True, you said they did not appear to leak, but you might not see the couple of drips that are all that is necessary to bleed off pressure.

2. If the disks are not perfectly straight, they will wobble back and forth a bit, spreading the pads apart. You will have a "mushy" or "soft" lever until the pads are squeezed back into contact with the disk.

.
 
Nessism said:
......... The tying the lever back trick is mostly to allow the caliper pistons to creep out of their bore closer to the disc, thus each lever pull won't have to push the piston as far (firming up the lever). Of course, if the caliper is full of crud all bets are off until you take it apart and clean everything properly. .......
Click to expand...

Caliper seals and the grooves they sit in are designed to roll back the pistons so there is a set clearance between the pads and the rotor. When the pads have slight contact with the rotor if there is any rotor runout they will abrade the rotor at the highest runout and develop thickness variation causing pulsation.

But I don't believe this premise is happening, at least once you go kickstand up. The other factor is rotor knockback unless you have a situation that I've rarely ever found of no rotor runout. With rotor runout each back and forth motion of the rotor hits the finger side and then the piston side of the caliper like a little hammer, and the repetitive forces eventually move the piston back so there is pad to rotor clearance equal to that of the rotor runout. Rotor knockback and seal rollback together are supposed to keep the pads off the rotor and prevent disc thickness and the resultant pulsations from occurring.

Now if the caliper slides are not free and the seal is not doing it's job due to heat hardening or crud buildup then you will eventually have pulsation. Or if runout is too excessive, which is why there is a spec for runout of all rotors.
 
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