Meaning of 'off one's brake fluid'

We got a problem with this slunt, Ingrid, again. She’s off her brake fluid. Banging her head against the wall.

Is there another meaning of 'brake fluid'?

The speaker is a prison officer. And there isn't any other context.

Solution 1:

Given the context you have cited it is likely that it means 'off her medication'.

Without brake fluid your vehicle would be out of control, with nothing to slow you down so that you can drive sensibly.

Ingrid is likely meant to be on medication which in some way 'slows her down', whether that is anti-psychotics, medication for ADHD etc. Those medications provide chemicals that the body is otherwise lacking in some way, as you top up brake fluid when there isn't enough to do the job, they are taken to top up your natural supplies.

This may be supported by this analogy used by Joseph M. Carver, Ph.D., Clinical Psychologist, in The “Chemical Imbalance” in Mental Health Problems

As we discovered more about neurotransmitters, we began to identify which neurotransmitters controlled certain bodily functions or which were related to certain emotional/psychiatric difficulties. Serotonin, a neurotransmitter, was found to be related to body temperature and the onset of sleep. Research also identified Serotonin as related to depression and later to a variety of mental health conditions such as anorexia and obsessive-compulsive disorder.

As research in neurotransmitters continued, studies between neurotransmitters and mental conditions revealed a strong connection between amounts of certain neurotransmitters in the brain and the presence of specific psychiatric conditions. Using an everyday example, our automobile operates by using a variety of fluids such as engine oil, transmission fluid, brake fluid, and coolant (anti-freeze). Every automobile has a way to measure the levels or amounts of each of these needed liquids such as the dipstick for oil and transmission fluid and marked indicators for anti-freeze and brake-fluid levels. Using our dipstick to measure engine oil, for an example, we can find our engine to be found one, two, or even three quarts low. After a recent oil change, the dipstick may also tell us that we have excessive oil in the engine. To work properly, all fluid levels must be in the “normal range” as indicated by the dipstick. When we receive a blood test, values of certain blood components are given with the “normal range” also provided, indicating if a blood chemical is below or above the average range.

Solution 2:

From Prison Slang Glossary

Brake Fluid: Psychiatric meds
(informal 'meds' = prescribed medication)

I think it's safe to assume the metaphoric allusion is to the fact that the relevant effect of such medication is tranquilizing mentally / physically hyperactive detainees (who need to be slowed / calmed down).

UK prison guards (and psychiatric nurses working in detention facilities for the criminally insane) also use the term liquid cosh to refer to major tranquillizers used to calm rebellious or ‘difficult’ prisoners.