89 fxstc all stock.

My rear brake light at first doesn't light at all. then as you pump the rear brake the light will eventually get brighter. I'm clueless.

 

Is this only happening with the lever or the peddle or both/either?

 

Check the socket and bulb base for corrosion.

 

As with all light problems, start by removing and checking the bulb. If in doubt replace it. If a new bulb does the same then we can start the detective work!

 

You have moisture in the system. Flush your rear brake system and you'l be fine....

 

It could be a hydraulic problem or it could be an electrical problem. Easiest way to find out is to bypass the rear brake switch with a shorting pin.
Follow the rear brake line until you find the rear brake switch. It will have two AMP plugs on it. Pull them off and carefully push in a straightened paper clip. It's the two wires you are shorting together not the switch itself. Make sure the paper clip isn't touching any other part of the bike and switch on the ignition. If the lamp is nice and bright you don't have a fault in your brake light circuit. Could need bleeding through or it may be a faulty switch.
If your brakes haven't been bled through in the last 5 years that needs doing anyway. When I say bleed them through, you need to keep bleeding until the new fluid comes out of the bleed nipple.

 

Water in the brake fluid won't cause this or did I miss the joke?

 

To be honest my gut tells me that it's a bad earth, frayed or broken wire, bad contact in the multi-plug, corroded bulb housing, corroded bulb. On a 1989 bike they are the usual suspects.
Would be interesting to see if this is a hydraulic problem. After decades messing with Harleys I'm still learning.

 

He says when he pumps it, it will light. Same exact thing happens to mine when moisture gets in the system over time. The cover on these rear master cylinders tend to warp allowing moisture in over time... I've had this happen twice to mine over the 15 years of owning my Softail....

 

Front brake (lever) is as bright as rudolf's nose. Pedal only is the issue. I will try to bypass the switch when I get home. If it is as bright as the lever I will flush brakes and if that doesn't work I'll swap out the switch. I'll keep you guys posted.