I have a Honda 300ex and I'm trying to put new brake pads on the rear. I can not for the life of me push the rear brake piston back into the caliper to make room for my new brake pads! Does anyone have any ideas for pushing the piston back into the caliber? Hit it with a hammer, push with a metal rod, formulate a small explosion!!! I'm going to put a little wd-40 on it to see If I can loosen it up. I'm just out of ideas.....

 

use a C clamp

 

When I tried to push the piston back in, no matter what I did it would not move. I put the old brake pads on and put it back together and the brakes work. If the piston was frozen would the brakes still work? Someone else suggested using a C clamp? What do you use?

 

I've done a lot of brake jobs on cars in my time, always use a C clamp! No reason the same won't work on a fourwheeler. In fact, it's practically impossible to get that piston back in with out a C clamp! And, in answer to your second question, if the brakes work, the piston is not stuck. Good luck! Jim.

 

Does it matter if the cap is still on the brake fluid resivoir?

 

Shouldn't matter if the cap is on the master cylinder. In fact, if you take the cap off, you risk getting air in the system, and you would then have to bleed the brakes at all wheels when you're finished. All you really need is the C clamp! Just be sure you are pushing the piston straight in and don't get "off center". Jim

 

NONONONONONONONONONONONONONONONONONONONONO!
The proper way to do this is to open the bleeder valve and then push the piston back in. Brake fluid will come out, catch it and dispose of it properly. If you are good, you will close the valve before the piston bottoms out so you will not get any air into the lines.
This is the correct way because if you force brake fluid back towards the resivour(which is exactly what everybody else is telling you to do) you will force fluid(dirty fluid at that) past the seals and o-rings that push fluid towards the piston. This will shorten the life of your caliper and master cylinder.
Now I know that people will tell you that I am full of bull because they have never experience any problems. This is bad info. One of the areas I am an expert on is automotive brakes. Fundamentally, a hydraulic braking system operates the same on any application. Basically you are moving fluid in one direction.
Autos and trucks have a few more features and devices in their systems, such and anti-lock hardware, proportioning valves, power boosters, etc, but the basics are the same. Just do yourself a favor and open the bleeder valve. You should also flush and refill your fluid at least once a year, more often if you operate in water which we have a tendency to do. That means bleeding your brakes.
One last thing to be carefull of. When bleeding brakes, be sure to not push the brake lever/pedal all the way in. the O-ring in the master cylinder wears a nice smooth area where it travels during normal operation. If you pust the O-ring into the area that is not worn, smooth you can shorten its life. That is not very common, but it is worth not finding out I am correct the hard way.
Sorry about the long post, but I hope it helps you get your atv safely back on the trail.

 

if the master cylinder is a '1 way street' as you describe..how does the fluid go from the resevoir to the lines?

 

No!
Don't push the piston with bleeder open!
THIS is the best way to get air in your system.
BTW, if you push the piston how could air get in your system? The only reason why there are caps is to forbid dirt to get in, not air.
This is the way to do it.
Open the cap. Take any clamp one jaw on the caliper (outside) and the other jaw on the piston. If you want, you can add some little stuff to protect your piston from scratching it...but it's not necessary. Once you feel it's solid, squeeze. It's no use squeezing hard 1 sec and releasing. Keep in mind it's oil, not water or air... it's very dense. It will take some time (like maybe 5 sec) to get your piston back to its "original" place. You should see the folw of oil coming up in the oil tank (I don't know if it has a special name..it's the little tank on top of the master cyl.).
The best squeezers are the...progrip clamps (I think that's their name -at least here in Quebec-). It's black plastic clamps with yellow pastic where it is in contact...if you can recognize what I'm talking about, get one, they work great.
After, if you want to bleed, you can.

If what I told you doesn't work, there's a problem somewhere.

 

Like I said, people will say I am full of it but I am telling you, do it the way I described and you will be in good shape in no time.
For the gentleman who wants to know how a master cylinder works, read on. There is a hole that is uncovered when the master cylinder is not depressed. This allows for the piston to slightly retract from the rotor so it doesn't drag and wear out pads. Fluid travels freely at this point. When you depress your brake lever or pedal, the O-ring on the shaft of the master cylinder first covers this hole, then pushes fluid towards the caliper. When you force fluid backwards, you are putting stress in the wrong direction and you can scratch the piston. This stuff will wear out parts faster than necessary.
If you open the bleeder valve, fluid will come out when you push the piston in. As for getting air in your lines, so what? Just bleed them. That is the way the system is designed. I hate to think how many people will not change brake fluid because they are afraid to bleed their brakes.
Dude, just go to the garage and try it my way, you will be amazed at how easy it is. Sometimes you might still need a clamp, but not very much pressure/force is required once you get it started. The great thing is brake fluid is water soluable, cleanup is a snap, just hose everything off. Make sure you catch as much of the old fluid as you can and dispose of it properly. Good luck.