172 Cup brake bias - rear brakes compensator fault? Not the MOT issue!!

[Rob Thomson]

Hello all,

I use my bog-standard Cup for hillclimbs and sprints. For obvious reasons it hasn't seen much use this year, but has done two events since lockdown and it's locking its rear brakes more than it used to. At Curborough on Saturday it tried to spit me into the barriers...



Got away quite lightly, just some repairable damage to the bumper.

Can the compensator fail? If so, is the one from ECP any good? Appreciate I'd need to lose the spring mechanism. https://www.eurocarparts.com/p/bosch-brake-regulator-130745277

Cheers, Rob.

 

[Ph1 Tom]

I used to run these. I found the balance pretty good and no complications with a compensatory.

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[Rob Thomson]

Those look a great idea... but I need to keep mine standard for the class regs.

Anyone got any experience with the ECP regulator, or any other ideas for what might be wrong with it?

Cheers.

 

[Rob Thomson]

Just to conclude this one in case it helps anyone else... Turns out Renault still sell the Cup compensator, for not much more than the ECP part linked above.

Now fitted the brakes feel much better. I haven't competed yet, but it feels much happier and more stable on the brakes.

 

[Martin Nilsson]

Hi Rob

sorry to reopen an old post but I’m also after a new compensator for my 172 cup but it doesn’t look like Renault are selling them. Do you happen to know the OEM 172 cup compensator part number?
If not is the Bosch Euro car parts regulator you linked the same as the one you got from Renault?
Also when fitting it to a non ABS cup do you just remove the spring from the lever arm and leave it floating?
The only information out there on the rear brakes is the MOT failure issue!

cheers
Martin

 

[Rob Thomson]

Hi Martin,

I’m afraid I don’t have the part number. Renault supplied directly to my local garage who fitted it.

The Bosch one looks identical except for the spring. I suppose it could be valved differently, but you’d think that’s unlikely. So yeah, just remove the spring…

Cheers, Rob.

 

[MicKPM]

8200248119 - Should be around £153+VAT at retail

 

[Martin Nilsson]

Cheers guys, job for the weekend sorted.

 

[thatflookguy]

Don’t suppose anyone has the Bosch number for the valve supplied by ECP do they?

 

[Rob Thomson]

Don’t suppose anyone has the Bosch number for the valve supplied by ECP do they?

130745277.

 

[thatflookguy]

130745277.

Ideal thank you. Will order one tomorrow as mine is FUBAR.

 

[thatflookguy]

Managed to find a new TRW version on the internet for £75 delivered. Nice to have fluid flowing through the rear calipers again

 

[Justinos]

Resurrecting an old thread - I have a 172 Cup which is locking the drivers side rear wheel under heavy breaking.

I don't have rusty discs like many suffer from, so I am definitely getting brake force to the rear - perhaps too much??

Could the compensator valve affect the force distribution to either side, eg could one wheel be getting more brake force than other due to the valve?

For completeness I have fitted new pads, replaced brake fluid, cleaned calipers etc, so running out of options to find the cause.

Could a weak master cylinder be an issue, I did notice that the passenger side front was hotter after a quick shakedown test, ie its diagonally opposite to the rear that is locking, but I'd just refitted the road pads, so maybe they were still re-bedding in.

At Cadwell, after a few warm up laps on Carbon Lorraine pads I never had the rears locking again all day no matter how hard I braked which was a big relief as it could have ruined the day, suggesting that the bias was more front with the more aggressive pad, but that doesn't explain only one rear locking not both.

 

[bashracing]

The brake system is a diagonal split so you can have more effort on one side to the other through the rear compensator if it's faulty, I would also look at bad corner weights if the car is on coilovers and hasn't been corner weighted and has considerably less load on the corner with the locking wheel.

 

[Justinos]

Its not on coil overs, so not corner weight related I would say.

I have ordered the compensator - Bosch from ECP - £50, cheap enough to replace and rule it out.

Could it be master cylinder related given uneven braking (possibly) on the 2 different diagonal circuits?

 

[16v_paddy]

You jammy b*****d!! 🤯🤯🤯 I desperately wanted to call bullshit on finding the compensator for £50 so went on ecp & there it is at £49.49 😲

Tried to buy 1 just because I previously paid £160 for the same one & it's out of stock 😭

I'd suggest having another go at bleeding the passenger rear caliper & really flush it through with loads of fluid, see how well the fluid comes out. I had an issue on mine where the calipers bled up fine (was using a handheld vacuum bleeder to do it) good solid pedal but still had no braking effort on the back.
The compensator was knackered & wasn't letting any fluid through it to the calipers, found this out by pulling all the fluid from the rear pipes & still had good pedal and the fronts working perfectly fine

 

[Justinos]

Sorry 16v_paddy - I normally prevaricate, but went for it!

So, have cleaned up the caliper slide pins, they were clean, just a bit dry.

Have power bleed all the calipers - it was newish fluid anyway - some micro bubbles, nothing major.

Fiddled with the compensator. Pulled the lever down to work the valve, free as anything, spring is holding it up at the furthest point, ie minimal rear braking if I understand correctly.

Now, the the corner weight comment has made me think. Been for a quick blast, came back and measured the disc temperature with a laser gun. The front passenger is 20-30 degrees hotter than the drivers side - rears not as much difference. This is diagonally opposite the rear that tends to lock - drivers rear. So wondering if the front passenger brake is biting more, so that corner dips, unloading the drivers rear, which will make it lock up easier??

The front brakes have been apart numerous times, new slide pins (was the first thing I did), pad changes etc, so nothing obvious that could be the issue if my theory is correct. Wondering if I just throw a new set of calipers at it, as I am now at the stage where I just start replacing parts until the problem goes away - its what the stealers do these days. They are TRW ones, where is a good source?

My theory is plausible until I remember at Cadwell, once the front Carbon Lorraines got up to temp I didn't have any lock ups or uneven braking all day. Perhaps with a higher caliper temp, the issue goes away.

 

[16v_paddy]

The spring & lever is supposed to be disconnected on a Cup, the braking effort to the rear is fixed which is the reason for the MOT bulletin thingy.

You can adjust the braking effort by undoing the big lock nut on the bottom of the compensator then wind the threaded bit in or out with a 5 or 6mm allen key but I doubt that will solve the unevenness side to side

 

[Justinos]

The lever is disconnected, the spring I refer to is only 15mm long, (#1) at the base of the lever that makes it return to vertical, ie minimum rear bias, the larger spring (#3) that is shown on the ECP part is not there as you would expect.

I wanted to work the valve in case it was sticking, bit of crud etc.

All this might be a red herring given my latest theory that the issue is the front brakes, not the rear!

The nut and allen key, is that #2? Which way do you turn to reduce rear effort?

 

[Rob Thomson]

When my compensator failed only one rear wheel was locking. The lever was free. Swapping it solved the issue.

 

[16v_paddy]

The lever is disconnected, the spring I refer to is only 15mm long, (#1) at the base of the lever that makes it return to vertical, ie minimum rear bias, the larger spring (#3) that is shown on the ECP part is not there as you would expect.

I wanted to work the valve in case it was sticking, bit of crud etc.

All this might be a red herring given my latest theory that the issue is the front brakes, not the rear!

The nut and allen key, is that #2? Which way do you turn to reduce rear effort?

View attachment 1686548

Aaahh gotcha! Yeah number 2 is what adjusts it, as for which way? I couldn't tell you as I haven't tried it yet. I was about to have a go at it trial & error style but ran into a problem of losing compression on cylinder 3 so been concentrating on fixing that & not bothered playing with the brakes

 

[Danith]

Does anyone know if this is just a thing on the 172 cup? I remember it being something to watch out for back in the day, but wasn't sure if it was on any other models?

 

[Ph1 Tom]

Only affects the Cup as it doesn't have ABS.

When I did the ABS delete on my 172 I used a Cup/Kangoo master cylinder and a pair of Ford Mondeo pressure reducing valves for the rear. Did away with the compensator completely and had much better rear braking.

 

[Justinos]

Aaahh gotcha! Yeah number 2 is what adjusts it, as for which way? I couldn't tell you as I haven't tried it yet. I was about to have a go at it trial & error style but ran into a problem of losing compression on cylinder 3 so been concentrating on fixing that & not bothered playing with the brakes

I would think winding out (lengthening it) the central allen key bolt will increase rear bias, and visa versa as pulling the lever down has the same effect, which I understand to increase rear bias.

I had the MOT done yesterday, the brake readings are suggesting that on the drivers side both front and rear have c 8% more brake force, but I am not sure how accurate those are, eg are the readings taken at the same time, or separate ones for left and right, which means there is the variable of how hard was the brake pedal pressed each time.

For now I will swap the compensator hoping I have the same issue as @Rob Thomson . If that doesn't do it I am then looking at Caliper refurbs/replacement in case a piston is sticking, then master cylinder as a last resort.

Tweaking the bias is also an option with the lock nut. At Cadwell with Carbon Lorraine Pads, once these were upto temp, ie more front brake performance, which is the same as less rear bias, I didn't have one lock up all day, or fade. At the end of the day I had clean rear discs and brake dust on the wheels. My 2 pals, one in another 172cup, the other in a phase 1 still had elements of rust on the rear discs. This leads me to think it is a rear bias issue.

 

[Ph1 Tom]

I would think winding out (lengthening it) the central allen key bolt will increase rear bias, and visa versa as pulling the lever down has the same effect, which I understand to increase rear bias.

I had the MOT done yesterday, the brake readings are suggesting that on the drivers side both front and rear have c 8% more brake force, but I am not sure how accurate those are, eg are the readings taken at the same time, or separate ones for left and right, which means there is the variable of how hard was the brake pedal pressed each time.

For now I will swap the compensator hoping I have the same issue as @Rob Thomson . If that doesn't do it I am then looking at Caliper refurbs/replacement in case a piston is sticking, then master cylinder as a last resort.

Tweaking the bias is also an option with the lock nut. At Cadwell with Carbon Lorraine Pads, once these were upto temp, ie more front brake performance, which is the same as less rear bias, I didn't have one lock up all day, or fade. At the end of the day I had clean rear discs and brake dust on the wheels. My 2 pals, one in another 172cup, the other in a phase 1 still had elements of rust on the rear discs. This leads me to think it is a rear bias issue.

That's why I changed to different valves for a slightly increased rear bias. Made a world of difference to braking and stability.