GT550k hissing sound when squeezing brake lever

[SClubSuzuki]

Hello folks, I rebuilt my brake system from top to bottom, including the master cylinder with a new piston kit. I’m having real trouble getting it to operate properly, after much bleeding the lever still goes all the way down without effect. I suspect the master cylinder bore may be shot because if I squeeze the brake lever quickly I hear a distinct hissing sound, like air is escaping. No fluid is leaking however, it’s tight as a drum. At first I thought it was the rubber dust boot that protects the piston, but I removed it and the hissing is still there. Unless I somehow installed the piston seal incorrectly? Any advice? Thanks in advance..

[Alan H]

Worst case scenario, just get a new master cylinder off ebay.
There are quite a few available that look similar, and will do the job ok for not many $$.
If you really must use the original, make sure that all the holes in the base of the m/c are perfectly clean and the castings are shiny and dirt free inside.

[SClubSuzuki]

Thanks Alan, I will disassemble it and double check. If all looks good I’ll go to eBay

[SClubSuzuki]

Haven’t torn into it yet but just realized that I used DOT 4 fluid instead of 3, wondering if there’s a viscosity difference that would cause my problem?

[karl pa]

DOT 4 shouldn't cause any difference from DOT 3, Sometimes the brake system just acts like that, if assembled correctly, try bleeding at the banjo bolt and get the master cylinder bled. then work down the lines.