I have noticed my 91 Chevy is lacking power in D and OD while going down the highway and when i tap my brake pedal my rpm's will shoot up suddenly from around 200-800rpms depending on the position of the throttle and how fast I am traveling but when it does this it is not shifting down, but instead just gaining the rpms could this possibly be linked to a defective brake booster? also my brake light is flashing 4 times and slow on the 4th flash idk if this is also related but I would also like to know what that is indicating

 

I had a 91 GMC Ex cab and from what I remember it had a vac operated brake booster. If the booster diaphram has a hole in it you will notice the engine change rpm at an idle for sure. I cant help you with the brake light flashing but if your unsure about repairing this your much better off getting a mechanic to look at it. Brakes are something you need!

 

well i have a replacement brake booster but i was just unsure if that could be the cause for my weird rpm fluctuation and im fairly capable of changing it but i was just curious if anyone had any input on anything else i may be overlooking but thank you for he help

 

If you disconnect the vac line and plug it and the rpm has no change when you step on the brake you will know your booster has a leak, Good luck with the booster change.

 

when you tap the brakes at highway speed the torque convertor clutch releases; there will be a increase in rpm. this is normal.

 

and it has a large loss of power in OD and at a stop while holding my brake the brakes will slowly bleed off so im leaning to a vac leak in the booster