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Brake Imbalance


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I've posted before about braking issues after fitting Wilwood calipers on all four corners. I've been persevering with this determined to get it sorted and avoid wasting a great deal of £s.

I've been advised that I should have around 800psi pressure at the rear calipers and around 1200psi at the front. This evening I checked this and find I'm getting 800psi front & rear. Could this suggest a faulty master cylinder? Master cylinder & servo were reconditioned last year.

P.S. vacuum at idle is 12inHg.

Any learned advice most welcome.

Richard.

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What exactly are you measuring Richard ?  If it's fluid pressure,  the pressure in a single hydraulic system is the same value everywhere.

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21 minutes ago, RobH said:

What exactly are you measuring Richard ?  If it's fluid pressure,  the pressure in a single hydraulic system is the same value everywhere.

Good point Rob.

I'm measuring the pressure at the calipers, bleed nipple removed and gauge fitted in it's place.

Richard.

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So your 800 psi is correct.   The only way it could be different front to back is if there were two completely separate hydraulic circuits each with its own master cylinder.  

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5 minutes ago, RobH said:

So your 800 psi is correct.   The only way it could be different front to back is if there were two completely separate hydraulic circuits each with its own master cylinder.  

Agreed. Then perhaps the 800psi is a little low, I may refit the repro servo, worth a try.

Richard.

 

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2 hours ago, RobH said:

So your 800 psi is correct.   The only way it could be different front to back is if there were two completely separate hydraulic circuits each with its own master cylinder.  

Some TR6's have in effect two master cylinders:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiO-r-g3f7vAhVHX30KHYLdCHgQFjAEegQIBBAD&url=https%3A%2F%2Fstatic1.squarespace.com%2Fstatic%2F5c6dec53b10f25d4edf0b3f7%2Ft%2F5c6ee9cb24a6942d310d0151%2F1550772684554%2FBrake%2BOverview%2B%26%2BTheory.pdf&usg=AOvVaw0vu_xUdZY5Q9yzQXQtRX8y

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Indeed Mike,  but the systems are not completely separate because of the interlinking of the two pistons. As it says in that piece:

"This means that the front and rear systems operate at the same pressure; otherwise the secondary piston would move in the direction of the lower pressure until the pressures equalize."

 

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If the secondary cylinder were sticking this could conceivably result in lower pressures to the rear calipers. The chart at the end of the article provides an idea of the booster performance . Given the problems with some repro boosters I wouldn't be rushing in to replace the booster.

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Is there not a slight difference in the piston areas? This would mean a difference in the pressure applied to each line. If you look at the cross section through the master there does appear to be a difference in the bore diameter. The force from the pedal is the same but the area it acts on is different - P= F x A.

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4 minutes ago, colin3511 said:

Is there not a slight difference in the piston areas?

There is a difference shown in bore and if the linkage between the pistons was solid it would certainly act as you say. However the spring between the pistons acts to equalise the pressures as described in the text in the second paragraph on the second page.. 

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What is the braking performance with 800 all round ? if the rears lock up before the front you would need to delve deeper. That advised 800/1200 split might apply to modern front wheel drive cars with a lot greater proportion of weight on the front tyres.

Peter

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56 minutes ago, CK's TR6 said:

Well, if you have to spend money, dual masters with a balance bar would eliminate the problem. works good.......

https://www.goodparts.com/product/dual-master-cylinder-kit-tr250-tr6-5-8-bore/

Thanks, I've already looked at that option. The extra expense surely isn't the answer to the braking issue I'm experiencing. 

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The standard m/cyl delivers less volume to rears than to the fronts, even when psi are same. With same-size (??) calipers all round the m/cyl rear-supplying piston may reach its max excursion without fully clamping the rear disc. Despite the psi measuring the same the rears may not exert much braking.

Peter

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19 minutes ago, Peter Cobbold said:

The standard m/cyl delivers less volume to rears than to the fronts, even when psi are same. With same-size (??) calipers all round the m/cyl rear-supplying piston may reach its max excursion without fully clamping the rear disc. Despite the psi measuring the same the rears may not exert much braking.

Peter

Thanks Peter, that's very informative and food for thought. Though even with my current set-up I should be getting far better braking performance than I'm experiencing right now.

I'm just about to bleed them yet again and hopefully get a test drive over the weekend, clutching at straws....

Richard.

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Hi Richard

are you really saying the calipers on the rear have the same piston size as in the front?

imho this would be unsafe!

the rears would lock up far too early under weight transfer during braking

what calipers do you have front and rear?

steve

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Hi Richard

Perhaps not what you want to hear but for what its worth I've found upgrading braking systems in then past usually requires two master cylinders and balance bar to achieve the set up you want. As said balance been the key to ensure the braking is predictable which may take a lot of experimentation to get right even with a dual MC set up. A lot of the kits are based around generic callipers to appeal to a wide range of applications. Rarely are they car specific and often not much better than a well maintained OEM set up with good pads/shoes and some simply add weight to the suspension.

Hope I'm wrong but you might have to face spending some more money to get the set up to work or go back to OEM unless your car sees the track on a regular basis.

Andy

 

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1 hour ago, Steves_TR6 said:

Hi Richard

are you really saying the calipers on the rear have the same piston size as in the front?

imho this would be unsafe!

the rears would lock up far too early under weight transfer during braking

what calipers do you have front and rear?

steve

Yes Steve, but the front calipers have 4 pistons and the rear have 2 pistons.

Richard.

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24 minutes ago, Richard71 said:

Yes Steve, but the front calipers have 4 pistons and the rear have 2 pistons.

Richard.

Richard Look at the size of the m/cyl reservoirs, the front one for the rear cyliderd is much smaller. Make sure however that you have not conneted the front brakes' pipe to the front of m/cylinder, the manual is unclear.

Peter

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1 minute ago, Peter Cobbold said:

Richard Look at the size of the m/cyl reservoirs, the front one for the rear cyliderd is much smaller. Make sure however that you have not conneted the front brakes' pipe to the front of m/cylinder, the manual is unclear.

Peter

Thanks Peter, I've definitely not made that error.

Richard.

 

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Richard, OK. Try checking the fluid level in the front reservoir with brakes actuated and see if you cna measure volume used. Then compare that with the volume available at max excursion of the piston in the m/cyl. Peter

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6 minutes ago, ntc said:

The reservoir size is only for capacity of fluid required the cylinder is the same. You could take it to mot and test the brake force on the rollers

Passed an MoT last year, even after the brake down-grade. I asked the tester if the braking was adequate, he answered in the affirmative. But as said, I don't feel they're as good as they should be.

Richard.

Edit...

I should add that I've covered over 2,000 miles with these brakes and without incident, but they really should be better or just as good as the standard set-up, not worse, hence my perseverance.

Edited by Richard71
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3 minutes ago, Peter Cobbold said:

Richard, OK. Try checking the fluid level in the front reservoir with brakes actuated and see if you cna measure volume used. Then compare that with the volume available at max excursion of the piston in the m/cyl. Peter

Good idea, though it might be simpler to re-fit a known good m/c which I removed during this venture.

Richard.

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