P38A Braking System Problem on P38a - Pedal Travel Question and What to do Next?

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Hello All,

So when I say master cylinder I mean the big thing with a reservoir on top that's not the ABS pump. I think the correct term for the unit is ABS modulator.

I took the old one apart today and what I found was the 2 master cylinders that feed the brakes have springs underneath them to return them to the "brakes off" position. These springs were heavily corroded and the fluid that drained out from around them was a silty brown sludge. There was a lot of sediment in it and although the springs hadn't completely failed they seemed corroded enough to have lost springiness.

The ABS modulator unit seems to have a chamber at the front that gets preassurised with the pump. This is pre-master cylinder chambers and sits below the reservoir. It is services by the front bleed screw. The two bleed screws at the back against the bulkhead bleed out each of the piston chambers in the master cylinder (2 as there are 2 brake circuits). My issue was that the pedal would depress only when bleeding the pressurised chamber but not the rear screws (master cylinder chambers) or calipers.

My guess is that either the springs had lost power such that they weren't opening up the passage between the pressurised chamber & master cylinders or that the sediment had got sucked back into the passages and blocked them up.

Either way the moral of this one is to change your brake fluid regularly, or at least on occasion! It's not something that I have ever really done as I thought a moisture issue would just manifest as spongy brakes as the fluid started to compress. Now, having seen how the springs deteriorated, I realise how with old fluid they were essentially just sitting in water.

Also, I had a go at removing the big allen key disky nut thing bit couldn't get it to budge, that where I decided to call it a day!
 
teflongrasshopper said:
Hello All,

So when I say master cylinder I mean the big thing with a reservoir on top that's not the ABS pump. I think the correct term for the unit is ABS modulator.

I took the old one apart today and what I found was the 2 master cylinders that feed the brakes have springs underneath them to return them to the "brakes off" position. These springs were heavily corroded and the fluid that drained out from around them was a silty brown sludge. There was a lot of sediment in it and although the springs hadn't completely failed they seemed corroded enough to have lost springiness.

The ABS modulator unit seems to have a chamber at the front that gets preassurised with the pump. This is pre-master cylinder chambers and sits below the reservoir. It is services by the front bleed screw. The two bleed screws at the back against the bulkhead bleed out each of the piston chambers in the master cylinder (2 as there are 2 brake circuits). My issue was that the pedal would depress only when bleeding the pressurised chamber but not the rear screws (master cylinder chambers) or calipers.

My guess is that either the springs had lost power such that they weren't opening up the passage between the pressurised chamber & master cylinders or that the sediment had got sucked back into the passages and blocked them up.

Either way the moral of this one is to change your brake fluid regularly, or at least on occasion! It's not something that I have ever really done as I thought a moisture issue would just manifest as spongy brakes as the fluid started to compress. Now, having seen how the springs deteriorated, I realise how with old fluid they were essentially just sitting in water.

Also, I had a go at removing the big allen key disky nut thing bit couldn't get it to budge, that where I decided to call it a day!
Click to expand...
Useful information. Many thanks
 
teflongrasshopper said:
Hello All,

So when I say master cylinder I mean the big thing with a reservoir on top that's not the ABS pump. I think the correct term for the unit is ABS modulator.

I took the old one apart today and what I found was the 2 master cylinders that feed the brakes have springs underneath them to return them to the "brakes off" position. These springs were heavily corroded and the fluid that drained out from around them was a silty brown sludge. There was a lot of sediment in it and although the springs hadn't completely failed they seemed corroded enough to have lost springiness.

The ABS modulator unit seems to have a chamber at the front that gets preassurised with the pump. This is pre-master cylinder chambers and sits below the reservoir. It is services by the front bleed screw. The two bleed screws at the back against the bulkhead bleed out each of the piston chambers in the master cylinder (2 as there are 2 brake circuits). My issue was that the pedal would depress only when bleeding the pressurised chamber but not the rear screws (master cylinder chambers) or calipers.

My guess is that either the springs had lost power such that they weren't opening up the passage between the pressurised chamber & master cylinders or that the sediment had got sucked back into the passages and blocked them up.

Either way the moral of this one is to change your brake fluid regularly, or at least on occasion! It's not something that I have ever really done as I thought a moisture issue would just manifest as spongy brakes as the fluid started to compress. Now, having seen how the springs deteriorated, I realise how with old fluid they were essentially just sitting in water.

Also, I had a go at removing the big allen key disky nut thing bit couldn't get it to budge, that where I decided to call it a day!
Click to expand...
Good info, how about photo's?
 
Datatek said:
Good info, how about photo's?
Click to expand...
Hello, A couple of photos attached showing the top and bottom of the cylinders. The springs in the bottom photo have the corrosion on them & when opened up that's where all the sludge was.
 

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OK, so a post-fix update/lesson learned.

After a couple of days driving around I started up the car one day to find the ABS and traction control lights remained on. This prompted some checks where I noticed that the brake lights were on regardless of the brake pedal being depressed on or not.

So the brake light switch operates by a knobbly thing extending out as the pedal is being depressed. It presses against a plate that meets it from the brake pedal. When the switch contacts the plate (brake pedal without a foot on it) it pushes it in and turns the lights off. After changing the master cylinder and clearly the dynamics of the pedal the plate no longer reached the switch to turn it off so it was permanently on.

Ultimately I think I need a new switch to fix this but bodged up a temporary fix with some cardboard and wiring loom tape wrapped around the contact plate to bring it up to the switch. I thought the wiring loom tape was a great option due to its abrasion resistance but, if i am honest, used it as it was the only tape I could find!

After sorting out the switch contact so the brake lights come on when the pedal was pressed I found this hadn't resolved the dash lights for ABS or TC. Luckily I have a Launch diagnostic tool which I bought a couple of years back to reset the steering sensor on my wife's car. I didn't think it would talk to the p38 but gave it a try and low and behold it could hook into the Wabco ABS module. There was a fault relating to no signal from the brake switch. I reset this and the lights went out. Have had a couple of long journeys since and looking good!

So the lesson learned is check you brake lights are working OK after fiddling with the brakes! Its a quick check and had I spotted this sooner I would have been safer and had a lot less hassle. I also think its worth a shout-out for the Launch diagnostic tools as I didn't think it would talk to the P38 but this cheep scanner could talk to the ABS and the ECU. I think its functionality is limited but it gives you the errors & lets you reset. I'm pretty impressed with it!

Also, I didn't try disconnecting the battery but it's possible this would have cleared the error code off the ABS module. I would be interested to know if anyone has cleared codes this way?
 
pwood999 said:
Take the switch out & reset it. Re-install while pedal is down, and then release pedal.
Click to expand...
How do you reset the switch? I fiddled around with it for a while but couldn't figure out a way to do it.

I stopped at opening it up as I wanted to make sure I had the brake lights working over xmas. Do I need to be braver?
 
teflongrasshopper said:
OK, so a post-fix update/lesson learned.

After a couple of days driving around I started up the car one day to find the ABS and traction control lights remained on. This prompted some checks where I noticed that the brake lights were on regardless of the brake pedal being depressed on or not.

So the brake light switch operates by a knobbly thing extending out as the pedal is being depressed. It presses against a plate that meets it from the brake pedal. When the switch contacts the plate (brake pedal without a foot on it) it pushes it in and turns the lights off. After changing the master cylinder and clearly the dynamics of the pedal the plate no longer reached the switch to turn it off so it was permanently on.

Ultimately I think I need a new switch to fix this but bodged up a temporary fix with some cardboard and wiring loom tape wrapped around the contact plate to bring it up to the switch. I thought the wiring loom tape was a great option due to its abrasion resistance but, if i am honest, used it as it was the only tape I could find!

After sorting out the switch contact so the brake lights come on when the pedal was pressed I found this hadn't resolved the dash lights for ABS or TC. Luckily I have a Launch diagnostic tool which I bought a couple of years back to reset the steering sensor on my wife's car. I didn't think it would talk to the p38 but gave it a try and low and behold it could hook into the Wabco ABS module. There was a fault relating to no signal from the brake switch. I reset this and the lights went out. Have had a couple of long journeys since and looking good!

So the lesson learned is check you brake lights are working OK after fiddling with the brakes! Its a quick check and had I spotted this sooner I would have been safer and had a lot less hassle. I also think its worth a shout-out for the Launch diagnostic tools as I didn't think it would talk to the P38 but this cheep scanner could talk to the ABS and the ECU. I think its functionality is limited but it gives you the errors & lets you reset. I'm pretty impressed with it!

Also, I didn't try disconnecting the battery but it's possible this would have cleared the error code off the ABS module. I would be interested to know if anyone has cleared codes this way?
Click to expand...

The plunger on the the brake switch can be set, you hold down the peddle and pull the plunger out then let the peddle come backup and it will set to the right distance.
 
brianp38dse said:
The switch is a 2 pole job, it needs to tell the ecu when the brakes are off and when on, and often the cause of an abs message on the dash
Click to expand...
And quite rightly so. If it hadn't triggered an error I would still be driving around with my brake lights on!

The p38's seem to be very well thought out.
 
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