Sorvall RC-5C doesn't get up to speed

The thread you referenced is for the 5B and you have a 5C, which may not use brush breakers. The fact that your motor turns at all indicates you have a closed circuit there.

Is the RPM / RCF switch set to show you RPM?

If the brushes light was on long enough to throw a Fault 4 (drive system failure) you may have damage to the motor. Did you use factory brushes to replace with? The brushes in that machine are not rectangular blocks that sit at right angles to the commutator, rather they are at an acute angle and the ends are shaped to fit the commutator curvature, giving a greater contact surface area. If you used a brush that is meant to sit at a right angle, even if the contact end is curved, you may not have enough contact and you may also be generating a lot of arcing. Did you check the commutator itself when you replaced the brushes? I've seen them aquire a tarnish coating that can interfere with speed regulation. It can be cleaned with a cleaning tool that you insert into the brush holder. Ideal makes a variety of them, and you can find them at industrial suppliers like Grainger, a motor / bearing shop, or possibly an electronics supplier. Search for Ideal commutator cleaner on the web. You may also get by with a swab dipped in alcohol.

Is the SS-34 your only rotor? Is it the only one with a speed issue? Are the rotor code switches set correctly for it? If the set temperature isn't proper for the rotor code and speed combination it might give you an issue.

Good point on the closed circuit.  The RPM switch is set to show RPM.  The brushes are from Helwig Carbon  part # 97-249921.  The brush ends are shaped to fit the commutator curvature.  The commutator looked to be in fair shape, with a bronze coloration (slight hue of purple, but not significant).  I did try a few q-tips with ethanol and was able to remove some carbon build up, however I was unable to reach a speed higher than 11,000 RPM.  I did try a GSA rotor (max speed ~13,000 RPM) and ended up with a fault 5 (overcurrent) after the rotor reached 9000 RPM.  The rotor code switches are set correctly for each rotor, and in the past these rotors have been fine running between 4 and 25 c for extended periods.  Thank you for the assistance!

What condition is the foam insulating disc in (under the rubber gyro cover in the chamber). If it's deteriorated there may have been cold and moisture drawn through the motor over time. I had to have my motor rebuilt once. I only had one RC-5C and going through my archives it looks like I had some speed control issues off and on. Some things I tried were cleaning circuit board edge connectors, speed setting wheel switches, and tach sensor, and calibrating motor current, top speed and slow start (in that order). I also had occasional problems with connectors - contacts loosening, wires breaking. It  ended up not being able to maintain low speeds properly, and was eventually replaced.

Post an email address and I'll send you the cal procedures.

clean the tachometer and adjust the same

Defective motor or Gyro is the issue. I,m a little late with this reply.  Sorry.