Stepped on brake hard to avoid crashing into car. Master cylinder hydro boost was leaking and pulled it out and a yellow plastic piece fell out and a small round metal piece with a seal type thing and don't know where they go
My car has 160000 miles.
My car has a manual transmission.
Small pieces such as the ones you describe are not readily illustrated in most automotive information systems, unfortunately I don't have access to detailed pictures with all the little pieces shown. This is a common challenge in auto repair, so don't feel as if you're being singled out here.
For the most part, what you have described doesn't seem to be anything that is going to stop you from installing a new hydro booster. Your new brake booster will come complete with everything it needs to be replaced, with one exception: you will need the clip that keeps the push rod installed. If this is broken or missing, you will need to get another from your local Ford dealer. The dealership will have access to engineering diagrams that list every single little piece on your car. In most cases you will be able to look at these diagrams with their parts professional and identify what it is that fell out.
In addition, you referred to it as a hydro boost. There were two different systems installed on that year Mustang. A hydro boost, which uses power steering hydraulics to give you the brake boost, and a vacuum booster that uses vacuum from your engine to aid in your brake boost. It will be important to know exactly which one you have when you visit your local dealer. A hydro boost system will have two or more hoses going to your power steering pump. A vacuum booster will not. It will only have one hose connecting to your intake manifold with a check valve mounted between them.
Long story short, I recommend visiting the parts department at your local dealership to identify the small unidentifiable pieces, although they may not be absolutely necessary for this job.
Good luck and be safe.
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