I believe you are experiencing the dreaded potentiometer problem (though it usually affects the brake). I couldn't find the original web page for the repair, but I have a text version:
Repairing Logitech Driving Force Pro Pedals
by PowerJunkie
This document will instruct you how to repair the "sticking" brake condition. This is a common problem and can be repaired very easily.
You can click on each picture for full size.
This is the tell-tale sign, the brake indicator staying on by one or two bars.
Tools you will need..........
* Phillips Screwdriver
* Needle Nose Pliers (any type of pliers will work, needle nose makes it easier)
* **Optional** Bar Clamp (such as the "Quick Grip") if no clamp, make sure you have someone to help hold the pedals
* table to work off of
* Rags or Paper towels
OK, now lets start.........
You will first clamp (or have your helper hold) the pedal assembly to the table with the pedals hanging down. By doing this it makes the job 5 times easier. Gravity is your friend.
Now using the Phillips screwdriver, remove the nine screws on the bottom as indicated.
Now gently pull of the bottom and set to the side.
WOW! Didn't know it could get that dirty inside there!
Now that we have access to the pedals, make note of the wires and their locations on the Potentiometers (the round gold pieces) The Potentiometers also have two tabs that fit into notches of the assembly case. One on top, one on the bottom.
Pay close attention to the orientation of the springs and how they fit into the grooves.
Lift the pedal straight up enough to slide the Potentiometer off. Once you have it off, check each wire by simply pulling on the connector. Do not pull on the wire itself! You will find that at least one wire is going to be loose. This is the cause of the brake sticking. This is where you will need the rags, the grease can be messy!
It is a safe bet to just go ahead and fix all three wires while you are in here. Pull one wire at a time off and using the needle nose pliers, GENTLY squeeze the connector closed until the end of the rolled edge just touches the flat back. Do this with both rolled edges. Using the pliers push the connector back on. It should go on very tightly. Make sure it is all the way on! Do this with all three wires. You can repeat this procedure for the Gas pedal if you want, as a safe bet.
Now, slide the Potentiometer back into the pedal and put the pedal back into the case making sure the Potentiometers tabs are in the notches and the springs is oriented properly. Carefully replace the bottom, it has the other notch for the Potentiometer, make sure it fits into it. Replace the nine screws and tighten (do not over tighten, this is just plastic!)
VIOLA! Now we are ready to race and not have to worry of a sticking brake no more!
