Lately and often I experience a momentary, very hard brake pedal at the end of a pull. It feels like a loss of brake booster vacuum. Car is a 93 third gen, stock brake booster, and plumbing to upper manifold. Generally only see the symptom when boost goes beyond upper teen/low 20's psi. I haven't noticed a check valve between the booster and manifold. Is there one I missed as I'm thinking that if such a part exists it may be worn out from 30 year age and handling higher boost. Another solution I've considered trying is creating a fitting pre-turbo as that should always have some vacuum. Anyone else, or has everyone else, had to fix this problem and what are some good techniques. Thanks.

 

if you have no check valve then that is your entire problem. you're pressurizing the booster with 20psi and then waiting for the vacuum of the engine to suck it all back down. in the factory hose that attaches to it, the check valve is built into it. you'll notice a little hump about half way through the hose. buy the check valve from an A70 supra or get a used oem hose if you don't have one at all or replaced the factory hose with something else.

 

I replaced that hose with OE sourced from Ray when I did a mechanical resto of my car 10 ish years ago. See if he still has any.

 

I replaced this hose which has the check valve built in, where the piece of tape is. The is a 93 USA car and the hose is the section connecting to the upper intake manifold. The check valve location of this hose was approximately 4 inches from the turbo so it was getting baked as well as seeing high boost. I created a new hose segment with a standalone check value and two sections of hose. The idea for this is to have an easily replaceable check valve and also to be able to locate it further from turbo heat. The brake feel is greatly improved, even in constant vacuum situations. I'm guessing the valve failed slowly and thus was not allowing brake booster vacuum to build well anywhere. Since the change was slow I didn't notice until it became bad. The kit one of the shops is selling would also likely be a good solution for this type of problem.