If braking how would the vehicle react with a missing left rear wheel cylinder seal or caliper seal?

A missing left rear wheel cylinder seal (in a drum brake system) or caliper seal (in a disc brake system) will result in a significant loss of braking pressure at that wheel. When braking, the vehicle will likely exhibit the following:

* Reduced braking effectiveness: The overall stopping distance will be longer.

* Pulling to the left: Since the left rear brake is compromised, the right rear and front brakes will do most of the work. This imbalance will cause the vehicle to pull noticeably to the left during braking. This pull will be most pronounced at higher speeds and harder braking applications.

* Uneven brake pad/shoe wear: The working brakes will wear out faster than usual.

* Possible brake fluid leak: You'll likely see brake fluid leaking from the affected wheel. This is a critical safety issue and should be addressed immediately.

In short, a missing seal in the left rear brake will make braking significantly less effective and dangerous, leading to a pull to the left during braking. It's a critical failure requiring immediate repair.