How can you tell which brake system your 91 caddy deville has...the jm4 or the j55?

You can't definitively tell the difference between a JM4 and J55 brake system on a 1991 Cadillac DeVille just by looking at it externally. Both systems used similar components, and visual inspection won't reveal the internal differences that define these systems (primarily differences in the ABS module and its control strategy).

To determine which system you have, you need to consult your vehicle's documentation, specifically:

* The Vehicle Identification Number (VIN) Decoder: A VIN decoder (many are available online) can often identify the specific brake system installed at the factory. This is the most reliable method.

* The Owner's Manual: While unlikely to explicitly state "JM4" or "J55," the manual *might* provide clues about the presence of ABS and the type of system through diagrams or descriptions.

* The Parts Catalog: If you have access to a Cadillac parts catalog for 1991, looking up your VIN or the brake components should reveal the correct system designation.

In short, visual inspection is insufficient. You must rely on documentation associated with your specific car to identify the brake system.