Speedometer Malfunction

My Dad has an 88 chevy with a 305. About a month ago the brake light began to flash and the speedometer went out. We looked through the manuals (this truck came with factory manuals when we bought if from my grandfather) and can not find a reason for it. It continues to happen every now and then and the brake light continues to flash. If you hit the dash board the speedometer and odometer will start working again but the brake light keeps flashing. (I know really techniqual solution). It has about 159,000 on it. Any help is greatly appreciated.

I just started experiencing this problem over the weekend (hit my knee on the cover for the elec panel and hit the front end hard going over some bumps) in my 1994 silverado. The parking brake lights up and the speedometer sits at 0.

I was told to lube up the parking brake cable where it is attached in the rear and at by the pedal (perhaps it's sticking slightly). Otherwise, a thorough check of the elec sys should help out.

Any other advice is appreciated.