Disc Brakes

[AlanJ]

Hi Guys,
Just recently there have been various posts about problems with disc brakes, ie, squeeking, overheating or not working as they should so it got me thinking especially as some of you might have read when the front wheel on my Tour 301 was powder coated the co coated the disc mounting area giving me loads of runout on the disc. Now as I said it got me thinking and the following is, when you pull in the lever you push the fluid to the piston in the calliper which in turn pushes the pads onto the disc, ok we all know that's what happens, but the bit I am interested in is what happens next. What stops the brakes from binding on the disc all the time? Most pistons have a fluid seal and a dust seal. Is it when you put lever pressure on the piston the seal drags a bit out of shape and when you relieve pressure it moves back to its natural shape thus releasing itself from pushing the pads, probably aided a bit by the dust seal or is it the small runout on the disc that knocks the pads off or is it a bit of both? Sorry guys, but it was just something that I thought somebody might know the answer too.
Take care All Alan.

[Blurredman]

I always thought the master cylinder is a vaccum, and with the fact you are pulling the lever against the vaccum, once released, the fluid gets sucked back up as the vaccum in the cylinder returns to a non-presurised state. This also pulls the piston back too slightly, and then the pad is free just to float around where it sits with marginal contact on the disc.

I had a lot of problem with over-heating, squeeking (really badly..) and binding brakes and it eventually went away what with constnatly disasembling it and re-assembling it. After a while, just out of interest I looked over the front wheel whilst in motion (around 50mph) and noticed it wobbled quite substantially and definately wasn't true, at least rim side..

So it's worth not overlooking how true to the wheel is. It might look fine when stationary but often when cornering is when you will find the squeeling/issues as the act of cornering puts a lot of pressure on the wheel.

[AlanJ]

Hi All,
The previous post (morning M) mentioned that it might be controlled by a vacuum in the system, so, that woke me up so I have just had a call with Powerhouse Automotive, the company that refurbished the calliper for my Honda and I am pleased to say according to them my thoughts were ok.It is the deflection of the piston seal when under pressure and then a return to its original form that allows the small amount of runout on the disc to push the pads off. It must be assumed that no runout on the disc is not a good thing as of course too much runout would not be good either. I suppose I should have asked Powerhouse what the ideal runout should be but there you go, I forgot.
Take care All Alan.

[AlanJ]

Hi All,
Looking at the disc runout I have just had another call with Powerhouse and they are saying that every bike is different you have to look at the recommendation by the relevant manufacturer. Hope that all helps if you have a problem with your front brake. Incedently according to Powerhouse zero runout is ok so it would appear that the disc may not play a big part in the system, I must say though that with too much runout it will be a problem on the travel on the handlebar lever, I know that from when I had bad runout after the paint job on my Saxon Tour 301 wheel.
Take care All Alan.

[DAVID THOMPSON]

the modern system is very reliable this causes a lack of maintenance
around the puck area
1 never clean the puck area or axle area with a pressure washer
2 do keep the rubber area seal clean be careful with what you clean it with
3 the master cylinder adjustment is important when it is at rest fluid in the line needs to be able to flow
back into the reservoir or the heat of a warm day can put your brake on and ruin the disc and rotor

4 keep the pins the pads move on clean
dave

[MZ in Oz]

Never,never just push the caliper piston back when fitting new pads, this will just forced dirt and grit back into the seal.
Remove the piston completely and clean with scotchbrite, pull out seals and clean around the groove before replacing the piston
and seals with a very light smear of silicon grease.
100,00 miles later my brakes are still operating as good as new.
If you do have run out on the disc this will push the pistons back and cause excess travel in the master cylinder and may cause the
need for a double pump to get the brake to grab.

[Blurredman]

Interesting to note the comments above, as my vaccum idea was just being thrown out there.

Indeed on cars with drum brakes, the shoe return springs pulling the shoes together against the brake cylinder is what pushes the fluid back to the master cylinder.

I realised this as one day one of my drums was seizing on, on my Peugeot 205. The flexible hose had swelled up on the inside preventing the springs from pushing the fluid back.

It's a good thing that I replaced the drums not long before, otherwise it would have been impossible to get the drum off given that it had such a huge lip on the edge over the shoes

[AlanJ]

Morning All,
I must say the MZ and this forum doesn't half get around a bit this last post has brought UK, USA and OZ together, not a bad sized triangle is it?
Keep going guys. Take care Alan.

[Retired1]

There is no vacuum in the master cylinder or caliper. As was said before the twisting of the caliper seals along with the release of brake pressure is what retracts the piston/brake pads. Dragging brakes are often caused by a clogged m/cyl. cap vent. The usual symptom is brake drag when the system heats up, there needs to be a vent to allow pressure from the expanding fluid and the air it displaces to go somewhere. If you look on the m/cyl. cap you will see a notch or small hole that opens to the rubber boot that goes on top of the fluid. This vent often gets clogged by corrosion of the alum. body or mud/dirt/wax.
A tell tale the vent is clogged is a low or soggy lever that firms up when the brakes are used and get warm, eventually the lever will get stiff and the brakes will drag. If the vent is clear drag is either crap in the m/cyl. clogging the return port or caliper pistons are sticking.

[Puffs]

Removed

[Guesi]

Not only expands the brake fluid when water is mixed to the fluid and it is boiling.
When the fluid boils it produces air bubbles (as everyone can see when water is boiling for cooking spaghetti ).
And air bubbles can be compressed, which leads to a failure of the brake when you pull the lever...
The pressure you give by the brake lever does not reach the caliper.
And you will notice that

[Puffs]

Removed

[DAVID THOMPSON]

on both my 95 Saxon Tour (500cc rotax) and my 2002 rt125 i have had to use a pump
and force new fluid up from the bottom they both would not expel the air by pumping with the lever

dave