[Brakes] Uncontrolled brake engagement

[davidjaller]

Hello there Berlingo experts! Hope that someone can shed some light on my confusions!

I have a family 1.6 petrol from 2005, with ABS, and drumbrakes in rear. On Swedish version of MOT, i got complaint on rosty brake lines, and a complaint that would translate to "ride brakes: uncontrolled engagement, rear, left and right - 'bites' ". The MOT guy first started talking about load regulating valve, but then realized that its not fitted on mine(I read this is because of ABS). Thats also all I can find on the internet regarding similar issues.

I dont know if he mannaged to trigger this behaviour by trying the brakes so extremely hard or something. I could feel it myself after the MOT, by braking hard from very low speed. It felt like rear brakes engaged more than the front ones, and it felt weird. Never notices this before the MOT, and today it feels fine for me again (i have cleaned the drums with breake cleaning spray).

I have now changed the brake lines and while doing that I got confused about the workings of this brake system.

At the place where I assume the regulator valve would sit, just above and in front of rear axel on right side, there is a chunk of steel where two lines comming in from front(from ABS module), and two line going out to left and right rear wheels. See images.(The photos where tkaen during changing of the brake lines, thats why the pipes are cut). No spring or nothing. The pipe nuts have different sizes. The one to left wheel and one of them going to front is larger(m12), and the one to right wheel and second one to front is smaller. Also the wholes in bottom of thread have diferent sizes corespondingly.

This is strange I thought to myself, why would the left and right wheel have different dimensions? Could it maybee be previous owner that did a mistake when reasembling after repair? And could it possibly be related to the uncontrolled engagement issue somehow? And what is the purpose of this part? I assume the proportion between front and rear breaking is done in ABS module? Could it be possible that is was bad luck and I could pass the next MOT?

In the end I made the new pipes to fit as it was before. (Unfortunately, since the different threading, cuting pipe and flare again would be needed for trying to swap the lines)

Anyone experienced something similar on ABS car, or know what this part is called, or know why it is connected in this way, or where to find a schematic diagram for the whole brake system? Any feedback is much appreaciated. Thanks!

[ffrenchie]

They're probably different to stop you connecting them the wrong way round. It doesn't look like it's been altered from factory.

[Sol]

I think the two different sizes are to ensure you can't mix up the two circuits, as if one fails, the other will still work but in diagonal fashion. Front left and rear right or vice versa, need to operate together. If they were the same pipe and thread size, you could mix them up and in a failure, have braking only on the same side of the car which would be deadly.

Maybe internal corrosion in the lines caused the weird behaviour, or braking very hard has moved something that was sticking, or the rear brake auto adjusters have been ratcheted up. I'd look at the rear shoe adjusters and back them off a bit.

[jj9]

The block the pipes go into is a Pressure Reducing Valve (PRV) this is different to a load compensator valve.
You have the PRV because there are drum brakes on the rear.
Some vehicles with ABS and drum brakes at the rear have a PRV some do not.

As the others have said, the fitting into the PRV & wheel cylinder are different sizes so they can only be routed correctly.

The strange brake behaviour you experienced after the test could have been down to the hard application of the brakes, maybe corrosion as Zion mentioned.

By changing the pipes you will now have clean brake fluid through the system so that may have been all it needed.

Try applying the brakes hard whilst stationary then see if you experience the behaviour again.

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[davidjaller]

They're probably different to stop you connecting them the wrong way round. It doesn't look like it's been altered from factory.

Ah! That makes total sense. Thanks for the replay!

[davidjaller]

I think the two different sizes are to ensure you can't mix up the two circuits, as if one fails, the other will still work but in diagonal fashion. Front left and rear right or vice versa, need to operate together. If they were the same pipe and thread size, you could mix them up and in a failure, have braking only on the same side of the car which would be deadly.

Maybe internal corrosion in the lines caused the weird behaviour, or braking very hard has moved something that was sticking, or the rear brake auto adjusters have been ratcheted up. I'd look at the rear shoe adjusters and back them off a bit.

This makes sense, its releaving to get this understood. Different dimensions not helping me though since I was cutting the pipes and had all the chances to screw it up anyways  But I was carefully noting the setup, so should be ok.
Iternal corrosion sounds reasonable, the pipes where quite bad and would explain that it was temporary. I will take a look at the adjusters. The drums came of easely though, does that mean the shoes are allready backed of enough?
Thank you for replying!

[davidjaller]

The block the pipes go into is a Pressure Reducing Valve (PRV) this is different to a load compensator valve.
You have the PRV because there are drum brakes on the rear.
Some vehicles with ABS and drum brakes at the rear have a PRV some do not.

As the others have said, the fitting into the PRV & wheel cylinder are different sizes so they can only be routed correctly.

The strange brake behaviour you experienced after the test could have been down to the hard application of the brakes, maybe corrosion as Zion mentioned.

By changing the pipes you will now have clean brake fluid through the system so that may have been all it needed.

Try applying the brakes hard whilst stationary then see if you experience the behaviour again.

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Thank you for clearing that up! Guess the PRV is not the cause of so much problems as the LCV is, thats why its hard to find pictures of this part online.

Thats good news for me if you and Zion is right. There is now plenty of freach fluid in the rear of the system, prob compleately flushed out.

I say I have braked quite very hard when testing after the repair, and seems ok, I will try some more times though before the next MOT.

Another question: Heynes manual says if ABS then bleeding most be done with pressurizing kit. Is that allways the case? Is it very likely that air has gotten in the ABS moduel after rear pipe change? I only replaced the rear half of the long pipes if that makes a difference. I bled in normal fashon, two people method, until no airbubbles, passed half a litre almost, making sure topping of the reservoir frequently. My GF says brakes feels fine, and as normal. But I'm thinking that pedal stops litle bit to close to floor on first push, then early on second push. The pedal stops stiff and the braking is fine though.

[Sol]

If you had Lexia or DiagBox, you could possibly run the abs system tests, with a rubber tube onto the bleed nipple furthest from the abs pump into a jar with clean fluid so the tube is submerged, and while making sure the reservoir never runs dry, the pump would bleed itself and the system as it runs. Any diagnostic kit that could initiate the abs actuation tests would work.

[davidjaller]

If you had Lexia or DiagBox, you could possibly run the abs system tests, with a rubber tube onto the bleed nipple furthest from the abs pump into a jar with clean fluid so the tube is submerged, and while making sure the reservoir never runs dry, the pump would bleed itself and the system as it runs. Any diagnostic kit that could initiate the abs actuation tests would work.

Hm interesting. I will think about investing in a Lexia, on Ali Express the price is quite allraight, Im sure it would be useful in other situations also.

[jj9]

The rear brakes are notoriously hard to bleed because of the way the pipes are routed (up hill / down hill) air can get trapped and the fluid doesn't move it as the air doesn't want to go downhill, the fluid just runs through the air pocket.
It's very unlikely that you have air in the ABS block so personally I don't think that's your problem.

Pressure bleeding can sometimes help as the fluid is pushed through at a greater rate so it has more chance of removing any trapped air, notice I say sometimes because it doesn't always work.
Best to leave it a week then try to bleed it again, any trapped may have shifted and you may get it out. 

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