SOHC.co.uk Forums > CB350/400
Front brake squealing
Orcade-Ian:
Surely K2-K6s post is still very much 'on topic'. The effects of a dragging pad causes the glazing which in most - but not all cases, ultimately causes the squeal. Although fitting a stainless steel piston prevents the corrosion starting, as we often see with the OEM plated version, it plays no part in preventing the build up of oxides under the outer diameter of the seal. Coming from the aircraft industry, stainless steel was NEVER put into contact with aluminium alloys without a barrier paste. The presence of brake fluid around the piston possibly helps in that respect. Regular maintenance is the only answer on this simple system.
Ian
Ken4004:
Really informative input from everyone, it good to see all the opinions on this forum and just shows that there is a broad knowledge base on this subject.
Knowledge is gained by trial and error and it’s always better to learn from other peoples mistakes thats where you get experience from.
Still have not had time road test my brakes do hopefully over the weekend but will post my findings.
I have stripped my brakes down completely and I can confirm that there was as log of evidence of corrosion between the piston and aluminium calliper body caused by water ingress which took quick a bit of cleaning up.
Initially when I was bleeding the brakes I did notice a leak around the piston seal but after a few more minutes it stopped do put this down to the seal bedding in, has not leaked since .
At first I thought that the seal groove was damaged by the corrosion and I would need to buy a as new calliper but all good now and good braking force .
McCabe-Thiele (Ted):
Interesting about the suspected fluid leak, most of my experience is on disc brakes on cars.
I have put new seal rings in calipers after using 1000 grit wet & dry to remove corrosion from the steel caliper walls with no leak when built up. I've assumed the pressure seals any minor imperfections.
I have always thought that when you release the pressure from the master cylinder that it's high spots on the disc that push the brake pad and hence the piston back. It appears that on these old bikes it's the piston seal that does the final pull back if I am understanding it correctly.
exvalvesetdabbler:
If you have the brakes bleed and the moving pad housing in your hand, just rest your finger on the piston and squeeze the lever a small fraction, you should feel the piston move out and then retract a fraction when you release.
Regards
Dave
TrickyMicky:
--- Quote from: McCabe-Thiele (Ted) on April 29, 2026, 09:25:40 PM ---Interesting about the suspected fluid leak, most of my experience is on disc brakes on cars.
I have put new seal rings in calipers after using 1000 grit wet & dry to remove corrosion from the steel caliper walls with no leak when built up. I've assumed the pressure seals any minor imperfections.
I have always thought that when you release the pressure from the master cylinder that it's high spots on the disc that push the brake pad and hence the piston back. It appears that on these old bikes it's the piston seal that does the final pull back if I am understanding it correctly.
--- End quote ---
Yes Ted, the seal is square in section, and when pressure is applied to move the piston outwards it distorts slightly. When the pressure is released the seal reverts to its normal shape, pulling the piston back slightly. If you trawl through You tube there are a few clips showing this in action. Rather like a hydraulic ratchet.
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