Doing a brake flush on a 2015 F-150 that calls for DOT 3.

Use the DOT 4. Here's why:

DOT 3, 4, and 5.1 are all glycol-based fluids and are mutually compatible. DOT 4 has a higher boiling point than DOT 3, which is fine—it won't hurt your system and may even be beneficial under heavy use. Mixing them is acceptable in a pinch, though a full flush with DOT 4 is ideal.

Do NOT use DOT 5. It's silicone-based, not glycol-based. Mixing DOT 5 with DOT 3/4 creates a gel-like substance that can clog your entire brake system—calipers, lines, ABS module. That’s a costly and dangerous mess. DOT 5 is also not recommended for ABS-equipped vehicles like your 2015 F-150, as it doesn’t handle the rapid pulsing of ABS as well.

Bottom line:

• ✅ DOT 4 = Safe, compatible, slightly better performance.
• ❌ DOT 5 = Incompatible, damaging if mixed, not ideal for ABS.

Proceed with DOT 4 and complete the flush properly. If you frequently work on brakes, consider stocking DOT 3 or 4 going forward.

No to both. Use only DOT 3.

DOT 4: Technically compatible (glycol-based), but do not use it. DOT 4 has a higher boiling point but absorbs moisture faster. More critically, Ford specifies DOT 3 for the 2015 F-150's seal chemistry and ABS module calibration. Using DOT 4 can accelerate seal degradation and alter pedal feel/boiling characteristics the system wasn't designed for. Ford TSB 14-0130 explicitly warns against substituting DOT 4 in DOT 3 systems.

DOT 5: Absolutely not. It is silicone-based and immiscible with glycol fluids (DOT 3/4/5.1). Mixing creates a gel that destroys the master cylinder, ABS pump, calipers, and wheel cylinders. Requires a full system teardown and replacement. DOT 5 also compresses more, causing a spongy pedal and potential brake failure.

Action: Buy DOT 3 (meeting FMVSS 116 / SAE J1703). Flush the system completely per Ford procedure (typically 2-3 quarts). Bleed sequence: RR, LR, RF, LF. Torque bleeder screws to 8-10 ft-lbs.