Bicycle Tutor

Hi,
I've recently resurrected an old bike from my dads shed. Everything is absolutely fine except for the rear brakes (v-brakes) - they have this style of arms http://www.customriders.com/prod_imgs/BRLD10337.jpg (different brand).
When there is no weight on the bike, they work 100% perfectly, sharp, accurate etc,... However, when I am riding the bike, they are pretty much ineffectual. I've tried changing the height/angle that the pad hits the wheel etc... and various other bits and bobs, all to no avail... I simply cannot understand this.
Any idea's or similar situations been encountered?

It depends what you mean by "ineffectual". If they are not moving properly, snapping back open when released, fitting the rim improperly, etc. that is one thing. If you mean they just don't stop well, that's another.
If they're not stopping well when you're on the bike:
- make sure everything it in proper alignment (which it sounds like you've done)
- clean the rim with some rubbing alcohol or another cleaner that won't leave ANY residue
- sand the outer surface of the brake shoes to take off a thin layer of rubber from where they hit the rim. Brake pads harden up over time. Sometime if you remove the hard outer surface, you get to softer rubber underneath. If they are real bad, you may need new shoes.
Note that your front brakes will always be stronger than your back. Note that the brakes feeling "sharp" when there is no weight on the bike makes sense. It's a lot easier to stop a 20lb bike that one with an extra 175lb (or whatever) on it.

I had this same problem with my fathers GT, he was having difficulty stoping and I did what Dave suggested about sanding down the brake pads and it solved my problem.

Thanks guys, I will clean the rim, and give the brake shoes a good sanding

I recently bought a second hand mountain bike which had been assembled by an idiot. OK I paid very little for it and, other than the assembly - it was brand new! One of the first things I noticed was exactly how Megalev describes the back brake problem. I also had difficulties adjusting the rear derailer with the bike mounted on a work-stand - it simply didn't change up or down properly on the road.
I have never had a bike with rear suspension before (hinged frame and damper technique) and I eventually realised that the split cable outers and various stop ends between brake shoes and the handlebar lever make for some problems with these frames, in that the frame adjusts with the weight of the rider and the cable can, in some cases, slacken so that the hand lever is spongy and quite ineffective. When you get off the bike the shoes are rubbing against the wheel rim, almost locking the wheel.
I cured the brake problem by making new brake cable-outers of optimum length and routed more directly. This sorted the braking. I'm still working on the gears, but I'm confident that the same solution will work for this also.

Right, I am no further forward. I think I am going to buy some different brake arms to be honest - normal as opposed to http://www.customriders.com/prod_imgs/BRLD10337.jpg