MGA and MGB Diagnosing a Brake System That Won't Relieve Pressure

Tech Tip: MGA and MGB 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 

1. 180-673 MGA 1500 Master cylinder
2. 180-758 MGA 1600 & MKII Master cylinder
3. 180-757 MGB Single Line system Master cylinder
4. 180-765 MGB Dual Line Non-servo Master cylinder
5. 180-738 MGB Dual line Servo assisted Master cylinder

2. Incorrect Pushrod Adjustment Between Pedal/Servo 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 or servo 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

MGA1500

1. 180-832 HOSE, FRONT, BRAKE qty 2
2. 180-832   HOSE, REAR, BRAKE qty 1

MGA 1600 & MKII

1. 180-892 HOSE, FRONT BRAKE qty 2
2. 180-832   HOSE, REAR, BRAKE qty 1

MGB (RD/GT to (c)394300)

1. 180-897 HOSE, FRONT, BRAKE qty 2
2. 180-835 HOSE, REAR, BRAKE qty 1

MGB RD (RD from (c)394301 on)

1. 180-887 HOSE, FRONT, BRAKE qty 2
2. 180-837 HOSE, REAR, BRAKE qty1

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 

MGB Front Calipers

1. 180-516 CALIPER ASSY, BRAKE, R/H qty 1
2. 180-526 CALIPER ASSY, BRAKE, L/H qty 1

MGA 1600 & MKII Calipers

1. 180-519 CALIPER ASSY, BRAKE, PAIR qty1

MGA Front Wheel Cylinders

1. 180-641 WHEEL CYLINDER, FRONT, R/H qty 2
2. 180-656 WHEEL CYLINDER, FRONT, L/H qty 2

5. Wheel Cylinder Seizure

Moisture contamination and corrosion can seize wheel cylinder pistons.

Fix: Replace components 

1. 180-075 Wheel cylinders qty 2 (MGA are not sided and are sold individually) 
2. 180-636 Wheel cylinders qty 2 (MGB RD All, MGB-GT TO (C)138400)
3. 180-646 Wheel cylinder Sold individually qty 2 (MGB-GT from (c)138401 on bore size 0.875” bore size) left and right hand use same cylinder.

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. 182-150 Brake shoe set (one set contains four shoes) MGA 1500 uses one set for the front, one set for the rear. MGA 1600 uses one set for the rear only.
2. 182-100 Brake shoe set qty 1 (MGB All, Rear)
3. 264-791 Brake drum (MGA 1500 front disc wheels only, all rear MGA wheel)
4. 264-780 Brake drum (MGA 1500 front wire wheels only)
5. 264-810 Brake drum (MGB UP TO 67 banjo type rear axle)
6. 264-821 Brake drum (MGB 68 on tubed type rear axle)

7. Brake Servo (Booster)

An incorrectly adjusted or internally faulty servo can prevent the master cylinder piston from fully returning.

Symptoms

• Grabby brakes.
• All four wheels affected.

Fix Replace Unit:

Replace servo.

1. Dual line system servo 182-190. Replacement unit for 1976 – 1980 Dual line systems only.

8. 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.