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Rear brake pressure reducing valve - recommendations pleases for IVA


Wavey

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With my IVA looming in just over 2 weeks, I've discovered my rear wheels lock-up before the fronts ūüėē

I have Sierra uprights with HiSpec Ultralite 4 callipers. I'm assuming 50/50 braking between the front and back - backs locking first because of the weight at the front . . .

 

I need a valve that is non-adjustable for IVA. I've read a few of the kit-car forums and the Fiesta one looks like it will do the job. Ebay has a few:-

 

https://www.ebay.co.uk/itm/Genuine-Ford-Focus-C-Max-Fiesta-MK5-Front-Brake-Pressure-Reducing-Valve-1469610/272376228296?epid=1509753686&hash=item3f6ae369c8:g:3pwAAOSwEtRb4q5T

 

But I would like to following any recommendations /experiences from club members - and the best location for fitting on a Westfield.

 

Would really appreciate any help . . .

 

David.

 

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Everything in the system needs to be balanced but the critical thing is that the area of the pads and the size of the disc acting upon them is not too large on the rear.   When braking hard

Imo it has to be some sort of inconsistency either in the components, whether that being varying piston sizes, pad materials, master cylinder effectivity. Or an inconsistency in the installation, blee

I am not advocating others to do this but all three of my cars that have gone through IVA or SVA I took the standard rear pads and using appropriate safety gear I routed the centre section of each rea

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Rush Motorsport

Can you get away with less good pads on the back?

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I have the same problem with a Westfield complete kit ( golf Hispec rears), got a retest at the end of the month as my rears locked up in the test and that was the only fault.   The factory suggest, bed in the fronts and  fit brand new pads to the rear then take on a trailer to the IVA.   Westfield say rear brakes bed in faster than fronts because they are smaller.   Speaking to Hispec they claim this is news to them, never heard of rears locking up before fronts.  My IVA tester said it was a well known problem with the current westfields and was the most common fail at the moment.  Just spent £26 on some EBC pads for rear.    

 

Factory advised against the fiesta pressure reducer, if you can find one.

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Steve (sdh2903)
17 hours ago, Two7 said:

 The factory suggest...... 

 

Which is just a total bodge. Whatever you do when your car is roadworthy and everything's bedded in please check its still not happening as if you have to hit the brakes hard on a twisty country lane the last thing you need is your A*** overtaking you. It won't end well. 

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Gary Taylor

I’ve used the Mazda proportioning valve on the last 2 builds and the brakes have been spot on.

like Steve says it’s more than just fixing for the day as the rears will bed in and bite when you least expect it and at the most inconvenient moment 

58E010E2-F2C2-4F9C-A438-1BEB28BB6B80.png

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Ian Kinder (Bagpuss) - Joint Peak District AO

Just to add some more facts to this topic. If you've got the same pad compound front and rear with the factory provided Hispecs and the factory master cylinder, I'm struggling to understand what's been observed by several members at IVA.

 

I trailed to IVA and had only done a brief (<5 mile ) road test prior to IVA.

 

Here's my IVA Brake test results:

 

image.png.5d61b720aabcc097385e734f0a0cc2a3.png

 

These results show a higher pedal pressure was required to lock the rears than the fronts 

 

Given the front callipers are 4 pot and have a bigger pad and disc, I fail to see how the back brakes can lock before the fronts, with the rears being two pot with a smaller pad and disc size. The Master cylinder isn't a 50:50 split either, so it just shouldn't happen!

 

Also, I've done >17,000 miles in 4 years with this brake set up and haven't been overtaken by my own back end when braking !

 

I've only built and IVA'd one Westfield so can't argue with the factories advice either, they've clearly built and IVA'd lots of cars!

 

Discuss.......................

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Steve (sdh2903)

Ian your asking a question that only the factory can really answer and just because its not happened to you doesn't mean its not happening. This isn't an isolated case. The factory advice worries me as its a way to pass on the day and not ensuring a safety critical system is performing as it should. I have never heard anyone ever say 'don't bed in the brakes' before a brake test. 

 

My thoughts for what they're worth. 

 

I think the standard issue master cylinder has changed, when I don't know but the standard cylinder used to be a girling one like this and I believe was somewhere around 18mm bore

 

20180712_170611-1512x2016.thumb.jpg.f33effb2e375d744bbfb79e3992bc688.jpg.a8fda9ce80619333903d6510ccbaa8e7.jpg

 

Now its a bigger bore one and seems to be an AP item. Could it be this is no longer a 60/40 split?  As I believe AP don't use a bias split on the 'upgrade' AP cylinder. 

 

Screenshot_20201010-101640_Chrome.thumb.jpg.a0370183ad43ccdfc8184bff824732b0.jpg

 

Secondly, the hispec rear calipers are available in 4 piston sizes. If you still have your set @Ian Kinder (Bagpuss) - Joint Peak District AO measure what size they are and maybe someone who has the current issue could measure theirs to see if hispec/Westfield are now supplying items with larger pistons fitted which would give a greater pedal movement and greater clamping force at the rears.

 

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Ian Kinder (Bagpuss) - Joint Peak District AO

Hi Steve,

 

Yes, the factory could/should know unless they've had a supplier change without being told!

 

Even with a 50:50 MC, I'd still expect the fronts to lock first:

 

Fronts have this choice

 

image.png.03b16cdf3fdcaba4cac02ffc68c49a64.png

 

With the pad area- image.png.53ed37dad56a91fd22cc4657c3fb5553.png

 

Rear's have this choice:

 

image.png.2ba843d8c3434e0aa49d1cdabb10a3b5.png

 

image.png.fa9a3a7845fab00b8f5b0754dea4d793.png

 

Using- http://www.brakepower.com/help_abc_27_PAC_t.htm

 

Worse case would be with 27.3mm diameter pistons in the front with 38mm pistons in the rear with a 50:50 spilt

 

Front axle (worse case) 

image.png.d681f86b4d7810c14d49f960bc2c22e8.png 

Rear axle (worse case)

image.png.94bb81b95673d91416ca7a97340888e8.png

 

So the rear piston total area is 96.7% of the fronts. Very close!

 

I was told in Feb- that my pistons should be:

 

4x34mm front

 

2x38mm rear

 

Hence my fronts should be:

 

image.png.5659e2123cda7ce29c08207e74479171.png

 

which then gives the rear total piston area at 62% of the front.

 

You'll note the great tool doesn't ask for pad surface area as this doesn't actually impact stopping power, it purely assists with heat dissipation and wear rate. The piston surface area is what's applying the braking force to the discs via the pads. Hence the difference in pad surface area front to rear can be ignored for this discussion.

 

Putting my IVA figures and the worse case front piston sizes into the tool give

 

(I've not entered these- image.png.ca9cb5cb1610aa41229736c60b0d1b63.png)

 

and left them at default as I don't know what they are in a Westfield.

 

 

 

image.png.d90a8aa1b3e76a8fbc9df863ac624d1e.png

 

Which suggests a bias away from the 50:50.

 

Changing the figures to what I believe I've got, shows:

 

image.png.8c77bbddcacff43f3fe9e6976bd614ef.png

 

As tool is for sizing the MC, I'm not too sure what to make of the results, though :bangshead:

 

My Hispec rear piston diameter is 38MM (added 18-10-2020).

 

 

 

 

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Steve (sdh2903)

Interesting calculations there Ian.

 

Id thought the rear pistons should have been smaller but after checking the golf ones are 38. And the hi spec can't go any bigger so rules that one out. 

 

23 minutes ago, Ian Kinder (Bagpuss) - Joint Peak District AO said:

Yes, the factory could/should know unless they've had a supplier change without being told!

 

The very fact that they have a 'procedure' to get around it would hint that this is an issue that they fully know about already. 

 

 

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Ian Kinder (Bagpuss) - Joint Peak District AO
20 hours ago, Steve (sdh2903) said:

Interesting calculations there Ian.

 

Id thought the rear pistons should have been smaller but after checking the golf ones are 38. And the hi spec can't go any bigger so rules that one out. 

 

If I recall, there is a choice of piston sizes on the golf rear, however I went for the 38mm. Given the brakepower.com tool suggests:

 

One sliding calliper piston is equal to two fixed callipers pistons of the same size- 

 

image.png.13a757594c63a08cf35582bf11d2813d.png

 

I've not noticed any reducing in braking since swapping to the golf rears.

 

20 hours ago, Steve (sdh2903) said:

The very fact that they have a 'procedure' to get around it would hint that this is an issue that they fully know about already. 

 

Yep. Won't we don't know is how much research they've done behind this advice. i.e. Have the checked piston sizes and master cylinder ratio's or just advised to bed fronts in only as a 'quick and dirty' solution.

 

I'm with you and I'd want confidence in what brake split I'd actually got.

 

@Two7 given you've plan to fit new rear pads. Can you measure you rear piston diameters and let us know. I'd also strongly suggest you measure the fronts too.

 

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Thanks for all the thought people are putting in to this.  I am currently trying to bed in the fronts on the little bit of access road in front of my house ( it is a roundabout so I can get up to about 15 mph and hit the brakes), still locking rears some of the time, but perhaps not as often, hopefully I won't hack off my neighbours too much.  After a session of say 5 brakes, the rears are hotter than the fronts  but that may be down to size of pad / disc etc.   Hope to take it to a local MOT garage ( braking lots on the way ), to see if my brake numbers have changed since the IVA see below.   I have attached a copy of my IVA brake test that mimics the MOT brake test, I trust this one as it comes out the machine and no adjustment for weights etc.    If I can see positive changes in the brake numbers at the MOT garage and I can avoid 20 mph rear lock ups I will believe the WF story that it is down to pads bedding in faster at the rear and it just needs time.   If there is no real improvement ,   I will try and measure the pistons when I change the pads, probably do that at a little garage near the IVA test centre on the day so they have virtually no bedding in, which feels wrong but not as wrong as WD40.   I kind of feel this is a catch 22 / chicken and egg test.    The brakes will be fine for the test and real life once bedded in, trouble is you can't bed them in properly prior to an IVA.   I take the IVA guys point of view, for a car this light, Drums on the rear are all you need.  

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Just in case anybody notices, I have balanced up the parking brake, it passed the IVA, but I wasn't happy, I found the setscrew on the handbrake caliper arm was too long and stopping the NS outer pad from engaging fully.  Simple fix, plan to get that checked at MOT station as well.  

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Ian Kinder (Bagpuss) - Joint Peak District AO
1 hour ago, Two7 said:

Thanks for all the thought people are putting in to this.  I am currently trying to bed in the fronts on the little bit of access road in front of my house ( it is a roundabout so I can get up to about 15 mph and hit the brakes), still locking rears some of the time, but perhaps not as often, hopefully I won't hack off my neighbours too much.  After a session of say 5 brakes, the rears are hotter than the fronts  but that may be down to size of pad / disc etc.   Hope to take it to a local MOT garage ( braking lots on the way ), to see if my brake numbers have changed since the IVA see below.   I have attached a copy of my IVA brake test that mimics the MOT brake test, I trust this one as it comes out the machine and no adjustment for weights etc.    If I can see positive changes in the brake numbers at the MOT garage and I can avoid 20 mph rear lock ups I will believe the WF story that it is down to pads bedding in faster at the rear and it just needs time.   If there is no real improvement ,   I will try and measure the pistons when I change the pads, probably do that at a little garage near the IVA test centre on the day so they have virtually no bedding in, which feels wrong but not as wrong as WD40.   I kind of feel this is a catch 22 / chicken and egg test.    The brakes will be fine for the test and real life once bedded in, trouble is you can't bed them in properly prior to an IVA.   I take the IVA guys point of view, for a car this light, Drums on the rear are all you need.  

 

 

It still doesn't sound right! What if you have to hit the brakes hard >15mph on the way to your MOT garage and end up backwards into a wall etc.

 

As said before I gave mine a 5-6 mile road test before IVA just to check the car I'd built actually drove! I didn't focus to much on the brakes, but certainly don't recall any rear lock up at any speed! 

 

I'd be checking piston sizes front & rear before driving it again.

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Steve (sdh2903)

As Ian says still doesn’t seem right, Have you tried re-bleeding the fronts? A bit of air in the front line could be the culprit. Try jacking the car up whilst doing it.

 

another one is to press the brake pedal hard and wedge it hard overnight with a piece of wood then rebleed the next morning.

 

if there’s no air in the system then i still reckon there’s an issue with either a component or spec of the brakes, 1 because the factory sound as though they know things are marginal and 2 if the Iva tester is suggesting it’s a common fail.

 

another option is to fit an uprated pad in the front to improve the fronts, something like mintex  1144 or ds2500.

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