One shoe won't snap into spd pedal

Once you have bled the air from the brake lines,clamp off the front brake hoses carefully.Does the pedal feel better now?
Then the pad /caliper fit is allowing too much play.Is the pedal the same?Then clamp the rear hose and try the pedal.Is it fine with the rear hose clamped?If so we now know the problem is at the rear brakes.One common low pedal rear brake cause is brake shoes that do not fit the drums.Remove the drums and look at the shoes,are they showing contact wear fully or just in the middle of the shoe?Remove a shoe and place it in the drum.Can you rock the shoe against the drum surface?Once drums have been resurfaced,the shoes will not fit fully against the drum allowing the shoe the flex when applying pressure to it and this can easily create a low soft brake pedal. Of course rear brake adjustment must be correct once brake shoe contact is correct.To correct brake shoe contact,have your shoes re arc-ed to fit resurfaced drums or install new drums.Don't overlook brake master cyl /brake pedal push rod adjustment too.

Being a mtn bike it probably had flat pedals or lock-in SPD. Either way, pedals dont come in sizes, the shoes do. A pair of pedals will fit ANY bike, just select the type you want. Just be careful when screwing on, there is a LEFT PEDAL and a RIGHT PEDAL. They screw on in oppposite directions.!

There are so many types of pedals. I recommend that you'll get a pair that you know and you are comfortable with. Or you can also check for reviews on pedals and take the ones with the highest rate. However, you should remember that what's fit some people will not necessarily fit you...
Check this link: http://www.vitalmtb.com/product/category/Flat-Pedals,20

All SPD pedals use a 2-hole side-by-side mounting pattern. Your shoes must have this same pattern or this is no way to mount the cleats.

Look at the bottom of your shoes:

Is there a 3 hole pattern in the shape of a triangle? Then this is a Road shoe, it may or may not have the 2 hole option. If it has the 2-hole option then you will see generally one fixed hole and a long slot next too it. If there is no threaded insert then one has too be added. Some road shoes that are capable of having the 2-hole option need an internal adapter.

If you have mountain bike shoes (shoes that the cleat once installed is recessed), then you should see two side by side holes. Again some shoes need an internal adapter to provide the threading to screw into. These should have come with your shoes. If you don't see any holes, then you may have a cover that needs removing to gain access.

If neither of these is your scenario then you should consider visiting a bike shop to have them look at your shoes and see what the exact problem is. Any half-way decent mechanic or shop person could tell you in a couple seconds if the shoes and pedals are compatible.

REMOVAL AND INSTALLATION

1. Remove disc brake caliper from mounting bracket and support with a length of wire or remove sleeve bolt and rotate caliper up and out of the way to remove shoes. Do not allow caliper to hang by the brake line unsupported.
2. Remove outboard shoe and lining. Use a suitable tool to disengage shoe springs from holes in caliper housing.
3. Remove inboard shoe and lining, unsnapping shoe spring from piston.
4. If installing new shoe and linings, bottom piston in caliper bore using large pliers. Take care not to damage piston or piston boot.
5. Remove bushings from mounting bolt holes in bracket. To install:
   
6. Install new bushings to mounting bolt holes in bracket. Lubricate bushings with silicone grease before installation.
7. Install inboard shoe and lining by snapping shoe retainer spring into piston. Shoe retainer spring is already staked to the inboard shoe. Shoe must lay flat against piston.
8. Install outboard shoe and lining by snapping shoe springs into holes in caliper housing. Wear sensor should be at the trailing edge of shoe during forward wheel rotation. Back of shoe must lay flat against caliper.
9. Install caliper.
10. Apply approximately 175 lbs. (778 N) of force 3 times to brake pedal to seat linings.

You adjust your cleats on your shoes, not the pedal.

TRy this http://www.valleybicycle.com/cleat%20adjustment.htm or Google "spd cleat adjustment"

Make sure the rear brakes ,if they are brake SHOES,that they are adjusted up,to where the drum drags on the shoes as you turn the drum by hand,the rear brakes,are what gives you pedal height.If this is fine,replace the master cylinder.

assuming your using Shimano pedal cleats?

You should have on the bottom of the shoe 4 threaded holes showing.
(if not then you will need to check the shoe box as sometimes they need to be fitted)

The cleat should be placed directly on top of this... with the point facing towards the front of the shoe.
You then need to place the OO plate on top of the cleat and place the crews in making sure the cleat is positioned straight.

Once tightened then place the cleat on the pedal and make sure its position is correct and your shoe is sitting straight then re-tighten.

hope this helps.

p.s. try here http://drivemybike.wordpress.com/2009/05/23/understanding-clipless-pedals-installing-cleats-on-shoes/

sounds like you have a worn set of park break shoes. they are located inside /behind the read rotors. if you remove the rotor and the shoes look fine then you can adjust them with the little star gear located on the bottom of the brake shoes.

if the booster is bad the pedal would be hard sounds like the e-brake is stuck take off the drums and make sure the arms are all the way back then did you adjust the rear brakes after changing the shoes