Modern Vespa

Anybody know what can possibly cause front wheels to lock up, the front brake lever to go stiff while riding?

I was riding around and all the sudden the bike locked up like that, to the point where I can't even turn the bike off and push it. I started the bike back up, let the bike rest for a bit, then the lever would loosen up slightly, and I tried revving real hard to overcome the brakes, sometimes it would allow me to do so for a while, and I'd be able to move the bike forward for a while, but as the bike decelerates, it would almost always start to stiffen and lock up entirely again.

Any clues what can be causing this problem?

I also hear some sort of small grinding noise in the front.

ABS unit failure?

aftermarket or left brake lever on the right handside

Other possible causes, extra to what Maksor says:

1- Brake fluid contaminated / overheated in some point and create a emulsion gel.
Easy way to check: if all returns to "normal" or is better with the brakes completely cold.
Solution: remove and replace all the brake fluid

2- Mix of DOT 5 (silicon base) brake fluid with the DOT 3-4 actual fluid. It generate a gel that leaves the calipers locked.
Easy way to check: only know if happens in a service or by mistake
Solution: remove and replace all the brake fluid (good flush is very important)

3- The level is installed wrong don't avoiding the pressure return (happens to me in the Ducati ones)
Easy way to check: remove the brake level, if the brakes become loose you are in
Solution: Install brake level correctly

One of your calipers is stuck. It's heating up and locking up.

did you wear the pads completely to the plates and weld the plates to the rotor?

perhaps open the bleeder valves and pull the brake lever a few times, letting out a portion of brake fluid, have rags handy in place, to catch the expelled fluid. tighten up the bleeder screws, check the b.f. level and fill up if low. Repeating several times may cure the issue

Good luck

My money's on a blocked/clogged compensation port in the master cylinder. If the front wheels free-up when you loosen the brake line on the master cylinder (relieving pressure), that will confirm the problem. If you have a bit of free play in the lever, it's a compensation port clog from dirty fluid, etc. If there's no play, you have a lever issue as mentioned by others. Good luck.

Any pics. or diagrams of the compensation port would be helpful

Thanks

The compensation port is a small hole in the top of the master cylinder bore. When you release the brake lever, the piston/seals retracts to uncover this hole, which releases any residual pressure in the brake line. If this hole is plugged or blocked (sometimes the piston rod is misadjusted), pressure can remain in the brake line, which causes the brakes to drag or lockup. The compensation port is also called a vent port or by-pass port. Hope this helps.

You can easily test the compensation port by removing a front wheel and see if if you can pry the inside brake pad in with a screwdriver away from the rotor, i.e. compressing the piston back into the caliper.

Pump the brakes to make sure the pads on both sides are seated against the rotors then see if you can pry one of the inside pads away from the rotor.

Terrific illustration plus advice on fixing faulty brakes.

The diagram sure removed a whole bunch of questions about how these things work.

Kinda miss the old cables though, where a spot of grease in the right spot could get things going again........

Many thanks for all the input.