MGA Master Cylinder Bench Bleed Method

Tech Tip: MGA Master Cylinder Bench Bleed Method

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For an MG MGA brake master cylinder, the cleanest and most reliable bench-bleeding method is usually a short-loop recirculation setup before installation. MGA systems are simple, but the master cylinder sits low and can trap air stubbornly if installed dry.

What you need

• Bench bleeding kit (recommended)
• Fresh brake fluid 
• A bench vise with soft jaws 
• Two short brake lines or bench bleed fittings 
• Small screwdriver or pushrod 
• Catch rags (brake fluid damages paint immediately) 

Setup

1. Clamp the master cylinder level in a vise. 
   • Don’t crush it; just snug enough to hold. 
   • Keep the reservoir level. 
2. Install temporary bleed tubes. 
   • Run lines from the outlet ports back up into the reservoir. 
   • Tube ends should stay submerged in fluid so air cannot re-enter. 
3. Fill the reservoir about 3/4 full. 

The proper bench-bleeding procedure is:

1. Fill the single reservoir. 
2. Bleed the brake outlet circuit. 
3. Then bleed the clutch outlet circuit. 

        4. Keep the shared reservoir topped up throughout.

Setup

• Clamp the cylinder level in a vise. 
• Install temporary fittings/tubes on: 
  • the brake outlet 
  • the clutch outlet
• Route both clear tubes back into the single reservoir opening so fluid recirculates. 

Next:

• bleed one side at a time, or 
• leave both recirculating simultaneously. 

Operate the pushrod slowly:

• short strokes initially 
• gradually increasing travel 

You’ll see bubbles emerge into the reservoir from whichever circuit still contains air.

Continue until:

• no bubbles emerge from either return tube 
• pedal/pushrod feel becomes firm 

One more MGA-specific nuance

The brake and clutch pistons are actuated differently inside the combined casting:

• The brake side uses the main pedal force 
• The clutch side uses a separate piston arrangement connected through the cross-shaft linkage 

So even though the reservoir is shared, trapped air in one circuit does not automatically purge the other.

Another helpful trick

Since the outlets on MGA masters can sit slightly lower than parts of the bores:

• gently tilt the nose upward while bleeding 
• lightly tap the casting 

This often frees stubborn trapped bubbles.

Important MGA-specific tips

• The MGA master cylinder has a relatively long bore and can trap air at the nose end. Tilting the front slightly upward while bleeding sometimes helps release the final bubbles. 
• Do not bottom the piston aggressively on an old cylinder; you can damage seals by pushing them into corroded bore areas. 
• If the cylinder was completely rebuilt dry, it may help to let it sit filled with fluid for 10–15 minutes before pumping. 

After installation

Even after bench bleeding, you’ll still need to:

1. Install the cylinder carefully without draining it 
2. Bleed the full hydraulic system at the wheels 

For an MGA, the normal order is:

1. Right rear 
2. Left rear 
3. Right front 
4. Left front 

(assuming left-hand drive)

Alternative method

Some MGA owners pre-fill the cylinder on the car using a pressure bleeder instead of bench bleeding. That works too, but bench bleeding is usually faster if the system is completely empty or freshly rebuilt.

One extra trick on these cars: lightly tapping the cylinder body with a screwdriver handle during bleeding can help dislodge tiny, trapped bubbles.