Clutch master cylinder

Clutch master cylinder refers to the hydraulic cylinder operated by the clutch pedal in manual-transmission Pontiac Fiero models.

The clutch master cylinder is part of the Clutch hydraulic system. It converts clutch pedal movement into hydraulic pressure, which is sent through the clutch hydraulic line to the Clutch slave cylinder at the transmission.

Overview

Manual-transmission Fieros use a hydraulic clutch system rather than a mechanical clutch cable.

When the driver presses the clutch pedal, the clutch master cylinder pushes fluid through the hydraulic line. That hydraulic pressure moves the slave cylinder, which operates the clutch release arm.

If the clutch master cylinder does not move enough fluid, the clutch may not fully disengage. This can cause hard shifting, grinding, or difficulty selecting gears.

A master-cylinder problem can feel like a slave-cylinder problem, air in the system, a clutch-pedal problem, or even a transmission problem.

Related topics:

• Clutch
• Clutch hydraulic system
• Clutch slave cylinder
• Clutch bleeding
• Clutch pedal
• Manual transmission

Location

Image wanted: Photo showing the clutch master cylinder location in a Pontiac Fiero, preferably with the reservoir, hydraulic line, and clutch pedal pushrod area visible.

The clutch master cylinder is located at the front of the car, near the clutch pedal area.

It is mounted to the front bulkhead/firewall area and is connected mechanically to the clutch pedal by a pushrod. Its hydraulic outlet connects to the clutch line running toward the rear of the car.

The clutch slave cylinder is located at the transmission, not at the pedal.

Function

The clutch master cylinder creates hydraulic pressure when the clutch pedal is pressed.

Basic operation:

1. The driver presses the clutch pedal.
2. The pedal moves the master cylinder pushrod.
3. The master-cylinder piston moves inside the bore.
4. Early piston movement closes the reservoir port.
5. Continued piston movement forces hydraulic fluid through the outlet line.
6. Fluid pressure moves the slave cylinder at the transmission.
7. The slave cylinder moves the clutch release arm.
8. The clutch releases.

When the clutch pedal is released, clutch spring pressure returns the hydraulic system toward its resting position. Near the end of the return stroke, the reservoir port opens again so fluid can move between the hydraulic system and reservoir.

Relationship to the slave cylinder

The master cylinder and slave cylinder work together.

The master cylinder creates pressure and fluid movement. The slave cylinder converts that hydraulic movement back into mechanical movement at the transmission.

A weak master cylinder can cause the same symptoms as a weak slave cylinder, air in the system, poor bleeding, or reduced pedal travel.

Related topic:

• Clutch slave cylinder

Hydraulic fluid

The clutch master cylinder uses brake fluid as part of the clutch hydraulic system.

Use DOT 3 brake fluid or equivalent brake fluid specified for the clutch hydraulic system.

Do not use mineral oil, paraffin-base oil, engine oil, automatic transmission fluid, or other non-brake-fluid lubricants in the clutch hydraulic system. Incorrect fluid can damage the rubber parts inside the master and slave cylinders.

Fluid level and condition are important. Low fluid can introduce air into the system. Contaminated or incorrect fluid can damage seals and cause hydraulic problems.

Related topics:

• Brake fluid
• Clutch hydraulic system
• Clutch bleeding

Common problems

Common clutch master cylinder problems include:

• External fluid leak
• Rear seal failure
• Internal seal bypass
• Center valve seal problem
• Worn bore
• Low fluid level
• Contaminated fluid
• Air entering the system
• Pushrod or pedal connection problems
• Incorrect replacement part
• Poor bleeding after replacement

A failing master cylinder may not always leak externally. Internal bypass can reduce hydraulic pressure without leaving an obvious puddle.

Symptoms

Possible symptoms of clutch master cylinder problems include:

• Clutch pedal feels soft or spongy
• Clutch pedal slowly sinks
• Clutch pedal does not return normally
• Clutch pedal travels to the floor with little resistance
• Clutch engages very close to the floor
• Hard shifting
• Grinding when selecting gears
• Difficulty selecting first gear
• Difficulty selecting reverse
• Car creeps forward with clutch pedal pressed
• Fluid level drops over time
• Fluid leak near the pedal area or front compartment
• Fluid near the master-cylinder dust cover
• Fluid on or near the clutch pedal
• Reservoir fluid level rises as the clutch pedal is depressed
• Problem improves temporarily after bleeding, then returns

These symptoms can also be caused by the slave cylinder, air in the line, clutch pedal problems, shift cable problems, or mechanical clutch problems.

External leaks

An external master-cylinder leak may show fluid near the master cylinder, reservoir, hydraulic line fitting, dust cover, or inside the car near the clutch pedal area.

Brake fluid can damage paint, so leaks should be cleaned carefully and repaired promptly.

If the fluid level drops repeatedly, the system should be inspected for leaks at both the master cylinder and slave cylinder.

Rear seal failure

A rear seal failure can allow fluid to appear around the master-cylinder dust cover or near the clutch pedal area.

Possible clues include:

• Fluid at the master-cylinder dust cover
• Fluid on or near the clutch pedal
• Fluid loss from the reservoir
• Pedal traveling to the floor with little resistance
• Air entering the hydraulic system

A rear seal failure requires master-cylinder service or replacement, followed by bleeding the clutch hydraulic system.

Internal bypass

A clutch master cylinder can fail internally.

Internal bypass occurs when fluid leaks past an internal seal rather than being pushed through the hydraulic line. This can reduce slave-cylinder travel without an obvious external leak.

Possible signs of internal bypass include:

• Soft or inconsistent pedal
• Pedal slowly sinks under pressure
• Insufficient slave-cylinder travel
• Shifting improves after pumping the pedal
• No obvious external leak
• Release arm moves at first but slowly returns while the clutch pedal is held down

Internal bypass can be difficult to diagnose because the system may appear dry from the outside.

A master cylinder can sometimes cause a clutch problem even when the slave cylinder has already been replaced. If the clutch works briefly after bleeding but then loses release travel again, the master cylinder should be considered.

Center valve seal problem

A useful diagnostic clue is the fluid level in the clutch master-cylinder reservoir.

If the clutch pedal travels to the floor with little resistance and the fluid level in the reservoir rises as the pedal is depressed, the master-cylinder center valve seal may be faulty.

This symptom points toward an internal master-cylinder problem rather than a simple need for bleeding.

Pedal and pushrod relationship

The clutch master cylinder depends on proper clutch pedal movement.

A bent or worn clutch pedal, worn pedal pivot, incorrect pushrod connection, or damaged pedal hardware can reduce master-cylinder travel.

If the master cylinder does not receive full pedal travel, the slave cylinder may not move far enough even if the hydraulic parts are good.

The clutch pushrod should remain properly aligned with the pedal bracket during operation. The clutch pedal lever and pivot pin should be square to the pedal, and the pedal pivot pin should fit the clutch master-cylinder pushrod snugly.

Related topic:

• Clutch pedal

Diagnosis

Basic diagnostic checks include:

1. Check clutch fluid level.
2. Inspect fluid condition.
3. Inspect the master cylinder for external leaks.
4. Inspect the master-cylinder dust cover.
5. Inspect inside the car near the clutch pedal for fluid leakage.
6. Inspect the pushrod and pedal connection.
7. Check clutch pedal travel.
8. Check pushrod alignment while the clutch pedal is applied.
9. Inspect the hydraulic line fitting at the master cylinder.
10. Check for air in the clutch hydraulic system.
11. Measure slave-cylinder movement.
12. Determine whether the master cylinder may be bypassing internally.
13. Have a helper press and hold the clutch pedal while observing slave-cylinder or release-arm movement.
14. If the release arm slowly returns while the pedal is still held down, suspect hydraulic pressure loss, internal bypass, leakage, or air in the system.
15. Watch the reservoir while the pedal is depressed; rising fluid level can indicate a center valve seal problem.

A clutch master-cylinder diagnosis should include the rest of the clutch hydraulic system. Replacing only one part may not fix the problem if there is also air in the system, a weak slave cylinder, a damaged line, or a pedal travel issue.

Hydraulic diagnosis clues

Symptom	Possible cause	Notes
Pedal travels to the floor with little or no resistance	No fluid, leaking pipe or hose, leaking connection, faulty master cylinder, or faulty slave cylinder	Inspect the full hydraulic system before bleeding
Pedal travels to the floor and fluid is found at the master-cylinder dust cover	Master-cylinder rear seal failure	Service or replace the master cylinder, then bleed the system
Pedal travels to the floor and reservoir fluid rises as the pedal is depressed	Master-cylinder center valve seal problem	Indicates likely internal master-cylinder fault
Fluid is found near the master cylinder and clutch pedal	Master-cylinder rear seal failure	Inspect the master cylinder and pedal area
Pedal feels spongy	Air in the system	Check fluid level, bleed, and inspect for the reason air entered
Pedal effort is high with long pedal travel	Incorrect master or slave cylinder size	Verify correct parts for the installed transmission and model year
Pedal effort and travel are normal, but gears cannot be selected	Clutch mechanism, shift system, or transmission problem	Check clutch, shifter, cables, and transmission diagnosis

Replacement

Replacing the clutch master cylinder generally involves disconnecting the hydraulic line, disconnecting the pedal pushrod, removing the mounting fasteners, installing the replacement cylinder, refilling the reservoir, and bleeding the clutch hydraulic system.

General replacement considerations:

• Protect painted surfaces from brake fluid.
• Match the replacement part to the correct model year and configuration.
• Inspect the pedal and pushrod connection.
• Avoid introducing dirt into the hydraulic system.
• Tighten hydraulic fittings and mounting hardware correctly.
• Bleed the system thoroughly after installation.
• Recheck for leaks after repair.
• Verify slave-cylinder movement after bleeding.

Related topic:

• Clutch bleeding

Bleeding after replacement

After replacing the clutch master cylinder, the hydraulic system must be bled.

Air trapped in the master cylinder, line, or slave cylinder can prevent full clutch release.

If the system is difficult to bleed, check for:

• Air trapped in the line
• Loose fittings
• Weak slave cylinder
• Incorrect bleeding procedure
• Master cylinder not fully primed
• Pedal or pushrod travel problems
• Damaged hydraulic line
• Damaged flexible hose

Related topic:

• Clutch bleeding

Relationship to shifting problems

A weak clutch master cylinder can cause shifting complaints that feel like transmission problems.

Common complaints include:

• “It shifts fine with the engine off, but not with the engine running.”
• “It grinds going into reverse.”
• “It is hard to get into first gear.”
• “The clutch pedal feels soft.”
• “Bleeding helped, but the problem came back.”

These symptoms should be diagnosed through the clutch hydraulic system before assuming the transmission itself is damaged.

Related topics:

• Grinding gears
• Manual transmission
• Clutch hydraulic system

Relationship to transmission type

The clutch master cylinder is used on manual-transmission Fieros.

Factory manual transmissions include:

• Muncie 4-speed
• Isuzu 5-speed
• Getrag 5-speed

The master cylinder is part of the car's clutch hydraulic system, but replacement parts and related hardware should still be matched to the model year and configuration.

Service notes

When working on the clutch master cylinder:

• Use DOT 3 brake fluid or equivalent specified brake fluid.
• Do not use mineral oil or paraffin-base oil.
• Protect paint from brake fluid.
• Inspect the clutch pedal and pushrod connection.
• Check pushrod alignment and pedal pivot fit.
• Check for leaks inside and outside the car.
• Bleed the system after replacement.
• Verify slave-cylinder travel after bleeding.
• Do not assume a new master cylinder automatically fixes all clutch-release problems.
• Inspect the slave cylinder if symptoms remain.
• Check the hydraulic line if the system repeatedly loses fluid or draws air.
• Recheck operation with the engine running.

Notes

A clutch master cylinder can fail without obvious external leakage.

When diagnosing Fiero clutch problems, the master cylinder, slave cylinder, hydraulic line, flexible hose sections, clutch pedal, pushrod connection, bleeding procedure, shift cables, and clutch assembly should be considered together.

See also

• Clutch
• Clutch hydraulic system
• Clutch slave cylinder
• Clutch bleeding
• Clutch pedal
• Manual transmission
• Muncie 4-speed
• Isuzu 5-speed
• Getrag 5-speed
• Grinding gears
• Shift cable
• Transmission identification