Thread: Spongy brake pedal

As with all power brake systems with the engine off you will only get a hard pedal, even on a brand new vehicle. Is problem spongy pedal or excessive play.

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Originally Posted by disco 3 door

As with all power brake systems with the engine off you will only get a hard pedal, even on a brand new vehicle. Is problem spongy pedal or excessive play.

Sent from my GT-N8020 using AULRO mobile app

Spongy - will slowly sink to the floor with engine running. Seems to be vacuum related. Vehicle has been manually bled, power bled and vacuum bled, new master cylinder, proportioning valve and the wheel bearings are ok. Brakes pump up hard with no engine running and sink a little when engine started.

A couple of other examples on this site.

http://www.aulro.com/afvb/discovery-...gy-brakes.html


Bleeding Defender Brakes - Any secrets ?

There are dozens of examples like this on the web. I just want to know how a vacuum pump/servo problem can cause spongy brakes.


Tom.

Hi Tom

Is your brake system ABS ?
.

Brake boosters have a small amount of slack between the pedal end and the master cylinder end, to operate the vacuum valve. Once the valve has been activated all the sponginess will be in the hydraulic system. A slowly sinking pedal is often a faulty seal on the master cyl secondary piston, allowing fluid to bypass the seal at low pedal pressures. Other than that I would be looking at flexible brake hoses for weeping/air entering the system.

Originally Posted by bee utey

Brake boosters have a small amount of slack between the pedal end and the master cylinder end, to operate the vacuum valve. Once the valve has been activated all the sponginess will be in the hydraulic system. A slowly sinking pedal is often a faulty seal on the master cyl secondary piston, allowing fluid to bypass the seal at low pedal pressures. Other than that I would be looking at flexible brake hoses for weeping/air entering the system.

I suspected there was a gap between the pushrods. The master cylinder is new and has been checked. I have new flex hoses for the front and caliper kits (pistons and seals). But the main problem is the sponginess. I am pretty sure that there is no air in the system. I will look at the servo and vacuum pump.

Now that I know that there is a gap between the pedal end and the MC end I feel a lot more confident that I can diagnose and fix it. I couldn't understand the problem as I thought there was no gap and I couldn't get my head around it.



Hi Arthur, it is ABS - I have replaced the ABS module too . I can actually get the ABS working if I can pump the brakes a few times.

Many thanks for advice - very helpful.

Tom.

Originally Posted by BathurstTom

Hi Arthur, it is ABS - I have replaced the ABS module too . I can actually get the ABS working if I can pump the brakes a few times.

Hi Tom.

My workhorse at the moment is ( 'Fred' ), a 300TDi man D1 with ABS, it will do until I get the Defender back on the road.

'Fred' has the exact same symptoms as yours with the spongy feeling pedal ( but only when the engine is running ) and I was loath to start repairing it from the wrong end first, so I have been watching with interest this thread and it's links to other threads.

From the descriptions on the other threads and as I do have a good spare master cylinder, that will be the first spot I am going to try.
.

Hi Arthur, the car has been to all four x 4 in Newcastle, and they tried another new master cylinder and it made no difference. I will be replacing the caliper seals and pistons and the front flex pipes and then looking at the vacuum system (pump and servo).

I am not inexperienced with brakes and this has had me foxed until bee-utey let me know that there is a gap between the pedal push rod and the master cylinder push rod. I know that when the engine is not running, the brakes will pump up hard and stay that way for some time, so I am confident in the hydraulic.


I have spent many hours reading of similar symptons on other land rover forumns, and I think that I can fix this now. As I am in Bathurst and the car is in Nelso Bay, it has been a long drawn process to get anything done, but I hope to get up there again in 2-3 weeks and go right over it.

I will let you know how I go.

Spongy pedal = air in system. Vacuum leak = hard pedal . When you pump the pedal about 5 times with engine off you will run out of vacuum. This is why when you start the engine the pedal becomes softer or spongy.

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