Soft Brake Pedal...

[Andy 308GTB]

Over the last few weeks the brake pedal has felt a bit soft.
This morning I removed all 4 wheels, attached the Gunson Eezibleed and bled all 4 corners.
Whilst the car was in the air, with the Eezibleed still in place, I hopped in the car, dabbed the brake pedal - solid as a rock. Happy!

I put everything back together & for safety's sake gave the brakes a dab - back to feeling soft.
When I push the pedal down, there is a slight hissing sound. If I stab the brake a few times it hardens up. But release the pedal and I'm back to a soft pedal.

I tested the brake fluid and it is <1% water - so is OK according to my meter.

I guess, the first thing to do is to flush the entire system with new fluid.
But I'm starting to think this is the master cylinder?

The fact that it was fine when the Eezibleed was in place will be key to the answer, I suspect.


2011 Mondeo MkIV

[gordmac]

If the engine isn't running a few pumps will use up the servo vacuum and the pedal will go hard.
Pump the pedal a couple of times and keep pushing on it, if the master cylinder seals are passing the pedal will slowly go down.

[Andy 308GTB]

gordmac said:

If the engine isn't running a few pumps will use up the servo vacuum and the pedal will go hard.
Pump the pedal a couple of times and keep pushing on it, if the master cylinder seals are passing the pedal will slowly go down.

Thanks for the reply.

With the engine off, if I pump the pedal it feels firm.
I keep pumping the pedal and it stays firm. If I then keep pushing, it doesn't really budge.

I switch the engine on and the pedal is soft, accompanied by a distinct hiss as the pedal sinks.
It does stop the car but it's definitely a soft pedal.

I'm still waiting for some fresh brake fluid to arrive but I'm not convinced a flush through is going to resolve this.
I've convinced myself its the brake servo leaking but I've found many theories on the Ford Forums!

[Chris32345]

If you press the pedal hard with the engine running it should be fairly hard and not move after going hard

[stevieturbo]

Andy 308GTB said:

Thanks for the reply.

With the engine off, if I pump the pedal it feels firm.
I keep pumping the pedal and it stays firm. If I then keep pushing, it doesn't really budge.

I switch the engine on and the pedal is soft, accompanied by a distinct hiss as the pedal sinks.
It does stop the car but it's definitely a soft pedal.

I'm still waiting for some fresh brake fluid to arrive but I'm not convinced a flush through is going to resolve this.
I've convinced myself its the brake servo leaking but I've found many theories on the Ford Forums!

If the servo was leaking, you would lose vac assistance and the pedal would be rock hard...same as when the engine is off.
You cannot ascertain pedal feel with an empty servo.

If the pedal is dropping with engine running/vac assistance, then there is either air in the system, a leak, or a leak internally like in the master cylinder.

But what lead you to doing this work in the first place ? Was there an actual problem before you started changing fluid ?
You say a soft pedal...air doesnt usually just randomly get into the system without some sort of cause or work being done.

Try clamping up one circuit with suitable hose clamps, to see if any softness is on one particular corner or circuit.

Are the pads in good condition ? I see an awful lot of pads falling apart these days, especially if a car sits about a lot that can give an odd pedal feel

Edited by stevieturbo on Sunday 5th April 09:41

[Andy 308GTB]

stevieturbo said:

Andy 308GTB said:

Thanks for the reply.

With the engine off, if I pump the pedal it feels firm.
I keep pumping the pedal and it stays firm. If I then keep pushing, it doesn't really budge.

I switch the engine on and the pedal is soft, accompanied by a distinct hiss as the pedal sinks.
It does stop the car but it's definitely a soft pedal.

I'm still waiting for some fresh brake fluid to arrive but I'm not convinced a flush through is going to resolve this.
I've convinced myself its the brake servo leaking but I've found many theories on the Ford Forums!

If the servo was leaking, you would lose vac assistance and the pedal would be rock hard...same as when the engine is off.
You cannot ascertain pedal feel with an empty servo.

If the pedal is dropping with engine running/vac assistance, then there is either air in the system, a leak, or a leak internally like in the master cylinder.

But what lead you to doing this work in the first place ? Was there an actual problem before you started changing fluid ?
You say a soft pedal...air doesnt usually just randomly get into the system without some sort of cause or work being done.

Try clamping up one circuit with suitable hose clamps, to see if any softness is on one particular corner or circuit.

Are the pads in good condition ? I see an awful lot of pads falling apart these days, especially if a car sits about a lot that can give an odd pedal feel

Edited by stevieturbo on Sunday 5th April 09:41

Thanks for this.
I think the pedal going soft has been a gradual thing, I took the car over from the wife and after a few months I thought that the brakes weren't great.
I had changed the rear disks and pads about a year previously, so I checked the front disks and pads - they were pretty worn so I changed them too. But the pedal was still soft.

Over the weekend. for my peace of mind and to save wasting time on here, I did a full flush of the brake fluid with brand new fluid - but nothing has changed.I checked the pads and disks and they looked mint.

So I am back to this:
With the engine off, a couple of pumps of the brake pedal causes the pedal to go hard.
With the engine running, the pedal is spongy and when pressed emits a hissing sound. It does stop the car well enough but it's not exactly confidence inspiring.

I'll try clamping off each corner - that's a good suggestion.

[RichB]

stevieturbo said:

Andy 308GTB said:

Thanks for the reply.

With the engine off, if I pump the pedal it feels firm.
I keep pumping the pedal and it stays firm. If I then keep pushing, it doesn't really budge.

I switch the engine on and the pedal is soft, accompanied by a distinct hiss as the pedal sinks.
It does stop the car but it's definitely a soft pedal.

I'm still waiting for some fresh brake fluid to arrive but I'm not convinced a flush through is going to resolve this.
I've convinced myself its the brake servo leaking but I've found many theories on the Ford Forums!

If the servo was leaking, you would lose vac assistance and the pedal would be rock hard...same as when the engine is off.
You cannot ascertain pedal feel with an empty servo.

If the pedal is dropping with engine running/vac assistance, then there is either air in the system, a leak, or a leak internally like in the master cylinder.

But what lead you to doing this work in the first place ? Was there an actual problem before you started changing fluid ?
You say a soft pedal...air doesnt usually just randomly get into the system without some sort of cause or work being done. Try clamping up one circuit with suitable hose clamps, to see if any softness is on one particular corner or circuit.

Are the pads in good condition ? I see an awful lot of pads falling apart these days, especially if a car sits about a lot that can give an odd pedal feel

Despite that, I reckon it's a faulty servo.

[Andy 308GTB]

RichB said:

Despite that, I reckon it's a faulty servo.

I really don't want it to be, I've just looked in my Haynes manual and it looks like a bit of a commitment (4 spanners on the Haynes scale )

[stevieturbo]

RichB said:

espite that, I reckon it's a faulty servo.

On what grounds ?

A servo failing to assist, the pedal would get hard.

I cant see any scenario where a failing servo would see a soft pedal ?

[RichB]

stevieturbo said:

RichB said:

espite that, I reckon it's a faulty servo.

On what grounds ? A servo failing to assist, the pedal would get hard. I cant see any scenario where a failing servo would see a soft pedal ?

Only on the grounds that both times this has happened to cars of mine, an MGC GT and my DB MkIII replacing components in the servo cured the issue. I know no more as I didn't do the work.

[Andy 308GTB]

Further delving in the Haynes manual, has offered this procedure for testing the servo:

1. With the engine off, depress the foot pedal several times to exhaust the vacuum.
2. Now start the engine, keeping the pedal firmly depressed.
3. As the engine starts, there should be a noticeable 'give' in the pedal as the vacuum builds up.
4. Allow the engine to run for 2 minutes, then switch it off
5. The brake pedal should now feel normal but further applications should result in the pedal feeling firmer, the pedal stroke decreasing with each application.

If the servo does not operate as described, first inspect the the servo unit check valve
If the servo unit still fails to operate satisfactorily, the fault lies with the unit itself. The servo unit cannot be repaired and must be replaced.

I think all of these steps have been suggested so far on this thread, one way or the other. But I'll go through this procedure shortly.

[stevieturbo]

RichB said:

nly on the grounds that both times this has happened to cars of mine, an MGC GT and my DB MkIII replacing components in the servo cured the issue. I know no more as I didn't do the work.

What components did you replace ? Servos are usually sealed non serviceable items.

More lokely you had an issue with the master cylinder...which is a totally different part, although it does bolt to the servo

[RichB]

stevieturbo said:

RichB said:

nly on the grounds that both times this has happened to cars of mine, an MGC GT and my DB MkIII replacing components in the servo cured the issue. I know no more as I didn't do the work.

What components did you replace ? Servos are usually sealed non serviceable items. More lokely you had an issue with the master cylinder...which is a totally different part, although it does bolt to the servo

It was not the Master Cylinder. p.s. I do know the difference between these components are and they are not "bolted to the servo" on the Aston (or the MG)

[crogthomas]

I just googled MGC brake servo and every picture that appeared showed the master cylinder bolted to the brake servo, except for the ones that did not have a brake servo.

[RichB]

crogthomas said:

I just googled MGC brake servo and every picture that appeared showed the master cylinder bolted to the brake servo, except for the ones that did not have a brake servo.

You are correct, I was wrong, in my defence the MGC GT was 40 years ago. The one fitted to my 1958 Aston Martin DB 2/4 MkIII is not connected to the brake master.

That said what was the point of this - whether they are or are not connected? I was simply trying to help the chap by offering an idea. As I said 3 posts ago I've had two cars with dodgy servos and soft pedal and that's all I know

Edited by RichB on Wednesday 22 April 18:19

[sliks]

stevieturbo said:

What components did you replace ? Servos are usually sealed non serviceable items.

More lokely you had an issue with the master cylinder...which is a totally different part, although it does bolt to the servo

They are sealed now but in the past diaphragm kits used to be available, and we're common.

Much like how hub assemblies have overtaken the wheel bearing in more recent times

[RichB]

sliks said:

They are sealed now but in the past diaphragm kits used to be available, and we're common.

Thank you Sliks, I thought I wasn't going mad, sittin here in isolation, now you've restored my faith that my brain cells are still working.

[289]

Andy 308GTB said:

So I am back to this:
With the engine off, a couple of pumps of the brake pedal causes the pedal to go hard.
With the engine running, the pedal is spongy and when pressed emits a hissing sound. It does stop the car well enough but it's not exactly confidence inspiring.

Any update?
What was the solution?

I have the same problem after bleeding.........hard pedal when engine off and sinks to floor when engine running as if there is still air in the system.

Interested

[Andy 308GTB]

289 said:

Andy 308GTB said:

So I am back to this:
With the engine off, a couple of pumps of the brake pedal causes the pedal to go hard.
With the engine running, the pedal is spongy and when pressed emits a hissing sound. It does stop the car well enough but it's not exactly confidence inspiring.

Any update?
What was the solution?

I have the same problem after bleeding.........hard pedal when engine off and sinks to floor when engine running as if there is still air in the system.

Interested

Hi,
I tested the servo per the Haynes Manual instructions

i.e.

1. With the engine off, depress the foot pedal several times to exhaust the vacuum.
2. Now start the engine, keeping the pedal firmly depressed.
3. As the engine starts, there should be a noticeable 'give' in the pedal as the vacuum builds up.
4. Allow the engine to run for 2 minutes, then switch it off
5. The brake pedal should now feel normal but further applications should result in the pedal feeling firmer, the pedal stroke decreasing with each application.

After running the engine for 2 minutes and switching off (Step #4). The pedal immediately felt solid - great. But when I started the car it felt soft again... I'm not sure if this indicates that the servo is at fault. However, at that point, I told myself that it's a fair size old bus and it does stop and it gets through an MOT - get over it.

It's still like it now, despite flushing the brake fluid again last week, which is a shame as I really enjoy driving this car. And a firm brake pedal would give me a lot more confidence.

[289]

I was hoping you found a solution.

As previously mentioned, I also feel a firm pedal after bleeding but puzzled as to why it sinks a fair amount after the engine is running.
I know the pedal sinks slighlty when running but shouldn't feel spongy or sink so far.

Mine also stops the car fine but doesn't lock any of the wheels
emergency braking. Something on my set up is wrong.

Will have another attempt with a different servo/master cylinder.