I forget the name of the part,Its like a master cylinder.It should be on the fire wall,under the hood.Its like a master cylinder except its a booster(about 8" around)Now that I think of it I think its called a brake pump booster assy.They used to make rebuild kits for this(but its not worth it and their not very reliable.Heres your best bet,go to the nearest dealer,have them look it up in the parts book(it shows the exploded view) its easy and its free.Since its a 91 they probably wont have it in stock,they only stock 4 years and back.Youll get a better idea of it and ask for the part number
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