Restoration guy brings in a 1999 Suburban with ABS and says he 'upgraded' it with DOT 5 silicone fluid for long-term storage.

Problem: DOT 5 silicone fluid is incompatible with the Suburban's ABS system. Here's why:

1. ABS Chattering: DOT 5 is silicone-based and doesn't mix with glycol-based fluids (DOT 3/4). It can trap air and create foam under the high-pressure cycling of ABS, causing erratic valve operation and chattering.

1. Spongy Pedal: Silicone fluid has different viscosity characteristics and can aerate more easily, leading to a spongy pedal feel. It also doesn't lubricate rubber seals the same way glycol fluids do, potentially causing seal issues.

Fix:

• Flush completely: Remove all DOT 5 from the entire system - master cylinder, lines, ABS module, calipers, and proportioning valve.
• Replace with DOT 3 or DOT 4: Use the correct glycol-based fluid specified for the vehicle.
• Bleed thoroughly: Use proper ABS bleeding procedures (may require scan tool activation of ABS valves).

Warning: Mixing DOT 5 with DOT 3/4 creates a gel-like substance that can permanently damage the ABS module and rubber components. This repair requires complete system replacement of all fluid and potentially damaged seals.

If the ABS module is contaminated, it may need replacement. Consider consulting a brake specialist - ABS module damage from fluid contamination can be expensive to repair.

You have a fundamental fluid incompatibility. DOT 5 (silicone) is not compatible with DOT 3/4 (glycol-ether) systems, and it destroys ABS modulators.

The Problems:

1. Spongy Pedal: Silicone fluid is highly compressible compared to glycol fluid. It also traps microscopic air bubbles that refuse to rise/bleed out. You cannot get a firm pedal with DOT 5 in a system designed for DOT 3/4.
2. ABS Chattering: The ABS modulator valves (solenoids) are calibrated for the viscosity and lubricity of DOT 3/4. Silicone fluid has different viscosity and zero lubricity. It causes solenoid valves to stick, chatter, or fail to seat, triggering false ABS activation at low speeds. It also attacks EPDM seals in the modulator not rated for silicone.
3. Hygroscopic Mismatch: DOT 5 doesn't absorb water. Any residual moisture in the system (from the old fluid) pools in low spots (calipers/wheel cylinders), causing localized corrosion and vapor lock under heat.

The Fix (Do this immediately):

1. Flush the entire system with fresh DOT 3 or DOT 4 (check cap/master cylinder for spec). Use a pressure bleeder or vacuum bleeder at all four corners.
2. Cycle the ABS modulator during bleeding (use a scan tool "Automated Bleed" function or drive on loose gravel to activate ABS pump) to purge silicone from the valve body.
3. Replace the master cylinder and ABS modulator if the chattering persists after a thorough flush. Silicone residue ruins the bore seals and solenoid seats.
4. Never use DOT 5 in a vehicle with ABS unless the manufacturer specifically specifies it (virtually no OEM does). DOT 5.1 is glycol-based and fine; DOT 5 is silicone.

Code Ref: FMVSS 116 (DOT 3/4/5 specs); SAE J1703/J1704. DOT 5 is for non-ABS classic cars only.