Spitfire Brake System That Won't Relieve Pressure

Tech Tip: Spitfire Diagnosing a Brake System That Won't Relieve Pressure

The procedures in this technical sheet involve hydraulic system testing, evaluation, and repair.

If you are unsure of any procedure, lack the proper tools, or are uncomfortable performing the work, do not proceed.

Improper testing or repair may result in vehicle damage, component failure, personal injury, or unsafe operating conditions. When in doubt, seek assistance from a qualified technician experienced with classic British sports cars.

The person performing the work assumes all responsibility for the use of the information contained in this document.

Condition

Grabby brakes, dragging brakes, or brakes that progressively lock up

This condition is almost always caused by hydraulic pressure that cannot return to the master cylinder reservoir after the brake pedal is released.

Why This Happens (Core Principle)

When the brake pedal is released, brake fluid must return through the hydraulic system and back into the master cylinder reservoir through the compensation (bypass) port.

Any restriction in this return path traps hydraulic pressure in the brake system. The trapped pressure keeps one or more brakes partially or fully applied until the pressure is relieved.

Symptoms

• Vehicle slows down without brake pedal application.
• Brakes become progressively tighter as the vehicle is driven.
• One or more wheels are difficult to rotate by hand after braking.
• Brake pedal remains firm and does not return normally.
• Brakes release temporarily after the vehicle sits for several minutes.
• Opening a bleeder screw causes a sudden release of pressure and frees the affected wheel(s).

Most Common Causes

• Blocked master cylinder compensation (bypass) port
• Incorrect pushrod adjustment between pedal/servo and master cylinder
• Collapsed flexible brake hoses
• Seized or sticking brake calipers
• Wheel cylinder seizure
• Rear drum brake misadjustment
• Brake servo (booster) malfunction
• Contaminated brake fluid

1. Blocked Master Cylinder Compensation Port

The compensation (bypass) port is a very small passage inside the master cylinder that allows brake fluid to return to the reservoir when the pedal is released.

Rust, deteriorated brake fluid, rubber debris, or sludge can block this port. If the master cylinder has recently been rebuilt, an incorrectly installed primary seal or excessive pushrod adjustment may also prevent the piston from uncovering the compensation port.

Symptoms: 

• Brakes remain applied after release.
• Brakes become tighter as the vehicle is driven.

Fix: Replace the Master Cylinder. Flush the system

• 580-100 Spitfire Mk3, MkIV & 1500 LHD thru 1975 (dual line system)
• 580-115 Spitfire 1976-80

2. Incorrect Pushrod Adjustment Between Pedal and Master Cylinder

A small amount of free play must exist between the pushrod and the master cylinder piston.

If the pushrod is adjusted too long, the piston cannot fully return and the compensation port remains covered.

Symptoms

Usually occurs after master cylinder replacement or rebuilding.

3. Collapsed Flexible Brake Hoses

Internally deteriorated brake hoses can act as one-way check valves.

Brake fluid flows toward the brakes but cannot return quickly after pedal release.

Symptoms

• One or more wheels drag.
• Brakes slowly release after sitting.
• Opening a bleeder screw immediately frees the brake.

Fix: Replace all flexible hoses even if they look fine from the outside

Front Hoses

1. 584-114 (1962-80) HOSE, FRONT, BRAKE qty 2

Rear Hoses

1. 584-045 (1962-72) HOSE, REAR, BRAKE qty 2
2. 021-168 (1973-80) HOSE, REAR, BRAKE qty 2

4. Seized or Sticky Calipers

Corrosion, contaminated brake fluid, or deteriorated piston seals prevent the pistons from retracting normally.

Symptoms

• Localized front brake drag.
• Uneven braking.

Fix: Replace components 

Mk1 & Mk2 Front Calipers

• 580-055 CALIPER ASSY, BRAKE, R/H qty 1
• 580-065 CALIPER ASSY, BRAKE, L/H qty 1

5. Wheel Cylinder Seizure

Moisture contamination and corrosion can seize wheel cylinder pistons.

Fix: Replace components

• 580-075 Wheel cylinders (3/4” bore) 1962-70 qty 2  
• 580-085 Wheel cylinders (5/8” bore) 1971-75 qty 2
• 580-090 Wheel cylinder (.70” bore) 1976-80 qty 2

6. Rear Drum Brake Misadjustment

Brake shoes adjusted too tightly maintain constant contact with the drum.

Symptoms

• Rear brakes grab immediately.
• Vehicle feels sluggish.
• Rear drums become excessively hot.
• Burning brake odor.

Diagnostic Testing

• Raise and safely support the rear axle.
• Release the parking brake.
• Rotate each wheel by hand.
• Inspect shoe adjustment.
• Remove drums if necessary.
• Check for continuous shoe contact.
• Inspect for glazing or overheating.

Fix: 

• Back off the brake adjuster until proper clearance is obtained according to Workshop manual specifications.
• Inspect brake linings and drums for heat damage, glazing, cracking, or scoring; replace components if required.
• Replace damaged or worn components

1. 585-320 Brake shoe set qty 1 rear only.
2. 586-530 Brake drum qty 2

7. Contaminated Brake Fluid

Old brake fluid absorbs moisture, causing corrosion, sludge formation and deterioration of rubber components.

These contaminants frequently block the master cylinder compensation port and contribute to sticking pistons and hose deterioration.

Final Verification

After completing any repair:

1. Bleed the hydraulic system if hydraulic components have been replaced or disconnected.
2. Verify correct brake pedal free play.
3. Confirm the brake pedal returns fully after each application.
4. Verify each wheel rotates freely after the brakes are released.
5. Road test the vehicle and confirm there is no brake drag, grabbing or progressive brake lock-up.
6. Check all wheels for excessive heat after the road test.
7. Inspect the hydraulic system for leaks and verify the brake fluid level before returning the vehicle to service.