Soft Front Brake Pull
- Thread starter glider.
- Start date
I found that my DOT 5 brake problem was exactly like stated above. The air bubble was trapped at the highest piston in the caliper. None of the indian tricks helped. I ended up removing the calipers and pads, pumping pistons out to full extension, removing master cylinder cover and slowly pushing the piston all the way back into the bores. I finally got to the last one (naturally) and when I forced the piston back in, a huge bubble came out of the master cylinder. Put it back together and brake is tight as it can be. If the bleed screw were on top of the caliper it would be real easy to bleed. I guess not as nearly as frustrating eh I mean fun.
I do believe over time that the DOT5 seems to develop air more than DOT4 ever did.
Bodeen
J
Jack Klarich
Guest
What, you can gravity bleed or power bleed these master cylinders, or bench bleed the old fashioned way
Gravity bleed is too "gentle" for those stubborn bubbles. Bordeen has the right idea if you can "level the playing field" by spreading things out. Have to admit, I considered doing that for the rear, but found feel was "good" enough after solo multiple bleeding attempts with the "heel and stretch" method AND "do or die failure/success method with a helper...LOL :newsmile011:
As for DOT 5 being less of a "performance" brake fluid, haven't seen any disadvantages other than harder to find, more expensive, and the fact silicone by nature is environmentally "terrible". It is like teflon, or certain plastics, does not break down in the environment and sticks to everything, being heavier than most fluids, it just settles to the bottom in globs. If you ever mix the stuff with glycol fluid you'll see what I am taling about. It does have a very high boiling point (hence the DOT 5 moniker) and does not attract moisure or breakdown like ethylene-glycol 3,4 or 5.1 fluids.
As for DOT 5 being less of a "performance" brake fluid, haven't seen any disadvantages other than harder to find, more expensive, and the fact silicone by nature is environmentally "terrible". It is like teflon, or certain plastics, does not break down in the environment and sticks to everything, being heavier than most fluids, it just settles to the bottom in globs. If you ever mix the stuff with glycol fluid you'll see what I am taling about. It does have a very high boiling point (hence the DOT 5 moniker) and does not attract moisure or breakdown like ethylene-glycol 3,4 or 5.1 fluids.
Last edited:
Can't wait to try it. I hope it fixes the problem:eam
J
Jack Klarich
Guest
Too much moisture in the brake fluid will give a spongy feel, if in doubt flush it with the proper type of fluid
RibEye
Senior Member
My front brakes are not spongy. Once the lever gets to the active point, the function is nice and hard. The problem is that it takes over half of the lever travel before compression is detected. I have rebuilt my master cylinder, and fully replaced all fluid during bleeding. Some improvement, but I did not see much wear on the MC seals. Before the rebuild, ineffective lever travel was around 2/3.
I'm told that accumulation on the caliper pistons can cause this. I can't for the life of me figure how that could be. Being an engineer, I need to know the failure mechanism. It seems that if the pistons were dirty, the pads would not retract, causing drag, but I can't see how that would account for the ineffective brake lever travel.
Last unrelated repair operation, the tech at the dealer did something I did not get charged for, that made the brake operation almost immediate in lever travel, but I did not get charged, nor did it get put on the invoice. I did seem to get quite a bit of brake drag in that state, until the lever travel worked its way back down a bit, over a few hundred miles.
I need a mental model of the failure mode. Can anybody help?
Enjoy,
Rich P
I'm told that accumulation on the caliper pistons can cause this. I can't for the life of me figure how that could be. Being an engineer, I need to know the failure mechanism. It seems that if the pistons were dirty, the pads would not retract, causing drag, but I can't see how that would account for the ineffective brake lever travel.
Last unrelated repair operation, the tech at the dealer did something I did not get charged for, that made the brake operation almost immediate in lever travel, but I did not get charged, nor did it get put on the invoice. I did seem to get quite a bit of brake drag in that state, until the lever travel worked its way back down a bit, over a few hundred miles.
I need a mental model of the failure mode. Can anybody help?
Enjoy,
Rich P
RibEye,
In my experience with a similar situation that you describe, I bled and re-bled, left the cap off over night with the lever tied, etc. I tried most tricks. I finally removed calipers from fork one at a time. I removed the pads and cleaned the pistons as you describe. I pulled the brake lever carefully using a block of wood so as to not allow the pistons to pop out. I cleaned them and removed master cylinder cover and pushed them back in. I found a huge air bubble behind the the top piston on the right hand caliper. When i forced piston back in, I could hear the bubble travel up to the MC. It helped a tremendously. My brake pull is still very good.
If you look at the bleed point on the caliper and the location of the top piston it would make sense that you could pump 10 gallons of brake fluid and never get rid of that bubble. trapped behind the piston and it is higher than the bleed point and out of the fluid flow.
Also.
This months American Iron Magazine's Hog Helpline article has a write-in question with a similar issue as ours with an 04 RK. The author states that many 03's and some 04's had a different master cylinder on them that causes the condition you describe. The fix is to replace MC with the newer style #45013-96D. I have not tried this but sounds logical. He states the bleed holes in the MC are different.
Hope that helps and let me/us know how it goes,
Bodeen