Boy, tough crowd today.
What 'peaked' my curiosity, was your comment, that OTHERS who have ridden your scoot, concur.
THAT says something 'bout the scoot, not so much you.
IF it were in my drive, here's what I'd do.

First, get a can of Brake Kleen, lift the scoot, spin the back wheel by hand, while spraying cleaner between pads and rotor. BOTH inside and outside pads. Also, pay close attention to the 'drag' as you spin the wheel.
NOW, before you do anything else, take it for a spin 'round the block. Any diff? No, then;
Pull the caliper, Remove the pads, take some 100 grit sandpaper, scuff sand the pads. Clean GOOD with Brake Kleen and blow dry (if you have a compressor)
Whilst they are off, use a 3M Scotch Brite scouring pad, CAREFULLY (unless you don't mind scratches on the non-braking area of rotor), clean the braking surface of the rotor. Again, clean thoroughly with Brake Kleen. (Basically, we are trying to determine 'contaminants')
Reinstall. Take it for a spin round block. Any diff?? No.
At this juncture, I would seriously tell you to get a new set of pads. I have seen this once in a GREAT while. Some pads, are just NOT condusive to certain rotors or rotor material.
As far as the master cylinder goes. It is nothing more than a reverse hydraulic cylinder. All it does, is develop a given pressure for the amount of force you apply. That is determined by the diameter of the piston. IE; 9/16, 5/8, 3/4. The larger the diameter, the MORE fluid you displace, at a LOWER pressure. IF you have changed your master, you indeed "could" be putting out more pressure than stock, BUT IMFUO, not likely. (Unless you replaced a 3/4 with a 5/8, or a 5/8 with a 9/16, etc.)
I'm focusing on the pads and rotor, right now, anyway. Given the info provided.
NOW, if it indeed turns out, you have upgraded the whole system, ALL bets are off, and we can go from there.
PLEEZ keep posting. ALWAYS wanting more 'knowledge', and this is how it sees daylight, and is shared.
THANX !!!!!!!!!
Glad you BOTH came thru in decent shape. WELKUM to the "Surviviors Club". Sorry you had to find out in front of Da Man.
I wear my leathers, regardless of temp, especially if long run or into 'strange' territory. For ALL you 'fashion cops', fingerless gloves would have helped alleviate the road rash on the hands. Saved mine more than once over the eons. Hard to "look really cool", unless your hands can grab the handlebars, and that is hard to do, with road rash all over them.
We have a living, breathing, posting example, right in front of us.

I saw 'proportioning valve' mentioned, also. No such critter, stock, there are 2 systems, not interconnected. Pretty hard to 'proportion' anything, in that scenario.
I have put proportioniing valves on scoots. They were installed for handicapped riders, who can only use their hands. One master cylinder, controls BOTH front and rear brake, at the same time.

Oh, and I ONLY know how to use the REAR brake.

One more 'side' thought. What are you running for fluid, 5 or 4???
For you guys running 4, something to keep in mind. "Most' manufacturers 'recommend' flushing the system, every 2 or 3 years.
I DO NOT believe that is your prob. BUT 4 is hygroscopic,(draws moisture from the air), this can cause 'gunk' to develop, which in turn, can cause probs in the system. One of the more common, brakes that 'stick on' or release s-l-o-w-l-y. (Yours sound like they are 'grabbing', NOT sticking. Just wanted to try to clarify the terminology I use) Another is 'spongy' brakes. The water, under the extreme pressure, will actually 'boil', causing 'steam' which, as we all know, is a 'vapor' and 'compressible', unlike fluid. (Think 'air' in the system)

 

Hey Gumpmeister. I did change the rotors to Harley floaters this last year but that is the only change and the rear brake has always been this way. It has stock pads on it. I was fortunately wearing some mechanics gloves and they helped but they weren't from a matching set and the left hand one was a little more beat up to begin with so it ripped open. I will be wearing my leather all the time again from now on. I thought I was just making a short trip on a hot day...

 

70% Front brake 30% rear brake.
slow, look, press and roll when negotiating curves. or do your braking before you enter the curve.
thanks Gump for your info

 

Spike, this isn't going to help much, but I sort of have the same issue, well not exactly the same.

I've mentined on here a few times about my disapointment in my front brakes on my Fat Bob. When I first got it, I was constantly locking up the rear wheel because the front brakes were just not doing much. I should add, they were not doing as much as my front brakes on my v-rod.

On my V-Rod I hardly ever used the rear brake because the front worked so well and would stop on a dime. Very impressive!

But it took me a few weeks to get the hang of the difference in the two bikes stopping ability. Now with the Fat Bob I MUST use both brakes. If I don't the front alone won't stop or the rear will usually lock up.

I've had my brakes checked, they are wroking normal. My buddy who also has a Fat Bob has the same complaints about his brakes.

Sorry I don't have a solution, but wanted to say that other riders also have issues with the brakes.

Glad you were not hurt too bad. What did the cop do?

 

Great post Gump! You always seem to have a good suggestion and I appreciate them. I'm sure many other people on here do too.

 

my turn again.
yes my rear brake will lock up if applied to strongly. as for your friends riding your bike are the metric croch rocket riders if so they don't know how to use a rear brake as all that I have ridden have little to no rear brake power as they use mostly front brake. yes I use mostly front brake on my softail also have learned how to use it in a corner.. my suggestion was not that you don't know how to ride maybe just need to hone some skills a little. front and rear brake together front only, rear only will skid out rear of bike. as to what gump said I to have been riding a long time and did the rear only no front brake one the bike been around a long time been riding for 50+ years and still say front and rear combo sill stop you fastest, in your case I probably would have just let off or one fingered the front brake .

 

Easy way to check and see if the brakes are dragging is take a ride of a few miles and don't use the rear brake at all. When you stop if the rotor is hot, you have a dragging brake.


Good luck.

 

Sorry about the spill. It can be a little scary getting back in the saddle, but you will get comfortable again. Sounds like your brake pads are sticking. I switched to Lyndall gold z plus brake pads. They reduced the amount of brake dust and because they are somewhat harder, they have a smoother engagement than the stock pads. I use my back brake alot when I am making slow speed u-turns and have not had a problem with the Lyndalls. On our newer Harleys, the pedal engagement is not adjustable. If you decide to get new rear pads, it would be advisable to flush the lines at the same time to get rid of any contaminents in the system. Hope to see you back on the road soon.

 

I think we have all been in a situation one time or another where we don't brake correctly with our bikes. I have had panic stops because I was watching the sidewalks instead of the car in front of me. I am guilty of not using the rear brake the way it should be, I tend to use the front brake more. (a habit I should break because I lose 30% of the stopping power) My new bike has the brembo brakes. I find these breaks are smooth and more responsive in a stop. Not sure if you have these on your bike, but I noticed a big difference.

 

Gump and spooner pretty much covered it, and some very good maintenance advise for all brakes.
One more thing if it hasn't been mentioned, make sure your brake line isnt pinched anywhere or have any kinks or blockages inside.
If its as bad as you say I would disasemble the entire rear brake system and start from scratch and blow out the line .
good luck