Eleven questions to ask before you place a trial order — covering bore and port verification, seal specification, leak-test evidence and what a real pressure-test report looks like.
Brake and clutch cylinders are unforgiving parts. A pad that is slightly off spec makes noise; a master cylinder that is slightly off spec loses fluid, and the customer loses their brakes. That is why the buyer's checklist for hydraulics looks different from the one for friction material.
Ask about the bore and the port, not the price
Everything that matters about a cylinder is dimensional: bore diameter, port thread size and pitch, mounting bore distance, pushrod length and reservoir interface. A serious supplier quotes those figures on the first reply, taken from a drawing rather than a catalogue photo. If the answer to “what is the bore” is “standard”, you are talking to a trader.
The second thing to ask is what seal material is fitted. EPDM and SBR are the correct families for DOT 3 and DOT 4 fluid; a supplier who cannot name the compound is buying on price and hoping.
The eleven questions
- Which factory produces this part number, and can I see its IATF 16949 or ISO 9001 certificate?
- What is the bore diameter, and what tolerance do you hold across the bore?
- What is the port thread and pitch, and does it match the OE unit exactly?
- Which seal compound is fitted, and is it compatible with DOT 3 and DOT 4 fluid?
- Is the body cast iron or aluminium, and how is it surface treated?
- Is every unit pressure and leak tested before packing, and can I see the test record format?
- Can you supply the matching left and right hand wheel cylinder as a pair?
- How do you inspect, at which stages, and can I see the report template now?
- What is the real MOQ per model, and is a mixed trial load possible?
- What is the warranty, and what happens if a unit leaks after arrival?
- Who is my single point of contact from inquiry to claim?
Look at the paperwork, not the website
A website can be built in a week. A leak-test record, a dimensional report and a loading video take an operating system to produce. Ask for all three before you ask for a price — the suppliers who can send them within a day are usually the ones worth your trial order.
Finally, run a small trial: one mixed load, a handful of part numbers, your own packaging. A cylinder either holds pressure or it does not, and one shipment will tell you more than a year of email.