Brake system-Refreshed and No pressure

[Tom W]

Unfortunately it’s not quite so straight forward, and just because others have moved the front cylinders to the rear and swapped master cylinders around when “upgrading” the brakes doesn’t necessarily mean it’s safe or wise.

You’ll need to do the maths to find out if you’ve changed the pedal ratio for the rear brakes by changing both the master and slave cylinders. It’s not a given that just because you’ve swapped both over from the front you have the same pedal ratio as the rears originally had, only that you have the same on the rear as the fronts originally had. Basically, if you’ve changed the ratio on the rear brakes, you’ve either made the brakes worse by reducing the travel necessary, or made the brakes better but increased pedal travel, possibly to the point where the system no longer functions safely in the event of circuit failure. If you have exactly the same ratio as before, then you’ve not actually changed anything for the modification.

You should also calculate the front pedal ratio, so you can see what you’ve done to the brake balance.

If the modified front master cylinder (in the event of a failure) requires more travel before it bottoms out and allows the rears to actuate, you’ll still run the risk of running out of pedal travel before you’ve properly actuated the rears.

[mtnjag]

Changed the Rear slaves from the stock 1 3/4” to the unused fronts at 2 1/8”. Master cylinder was not changed, just rebuilt. It’s stock for both front and rear pistons.

The change over to a 3/4” MC and Wilwoods for the front was done long time ago and all was working well until the rebuild of existing MCs and changeover to the 2 1/8” to the rear.

So you feel the 2 1/8 pistons at rear could be the problem?

[Tom W]

A few things to note.

You originally had a seized rear wheel cylinder. If so, then just by freeing that up you’ll need more pedal travel. You were previously only pumping enough fluid to move 3 cylinders, now you need to move 4.

Now you’ve moved the original front caliper pistons from the front to the rear, but retained the standard size master cylinder, you’ve increased the pedal travel required to operate to rear brakes.

As for whether the front master cylinder is appropriate in either bore or stroke to suit the Willwood calipers is unknown.

I strongly suggest that you either put the whole system back to standard specification. Alternatively, take it somewhere that knows how to size everything properly, and will stand by their decisions and work should the worst happen. Whether the original modification done to the fronts is safe is unknown too. Whilst it might appear to work whilst it’s all working, it sounds like you don’t have the failsafe functionality of a split circuit system. What the modifications have done to the brake balance is also unknown.

[mgcjag]

Great information Tom.....moveing calipers from front to upgrade the rear has been mentioned on the forum for S1 cars.....not sure if it was in relation to the 3.8 with kelsey hays system.....as you say...needs investigating...Steve

[mtnjag]

All good points.

So to confirm the system is not just a bias linkage but should result in autonomous braking by the alternate MC if one should fail?

[abowie]

Have a look here.

viewtopic.php?f=4&t=4120&hilit=kelsey+hayes

[mtnjag]

Thanks I’ll read through it when family leaves.

I note the two different cylinders as I have the original Dunlop MCs. They accomplish this by using different pistons, one being longer than the other, same body.

I am using the long travel piston for the 2 1/8” I moved to rear, but of course am using a 3/4”MC up front which may be screwing things up when combined with the larger slaves.

However, all the aftermarket MCs are the short piston/long travel as far as I can determine. I have two and took a look and suppliers seem to only sell one cylinder and you apply it to the rear or front by changing the rod from Clovis to threaded as needed. Some do sell a front and rear with the different rods but I asked and they are the same cylinder.

[Tom W]

So to confirm the system is not just a bias linkage but should result in autonomous braking by the alternate MC if one should fail?

Unfortunately that’s wrong. There is nothing autonomous about it. What happens on the standard system is in the event of one circuit failing is the other should remain completely functional, although the pedal travel required to operate the brakes will be increased.

To achieve this, the master cylinder piston on the failed circuit is pushed to the end of its travel. This now provides a rigid point for the end of the balance bar to react on, so force can be transferred to the working master cylinder piston. On the standard system with the correct piston travels, all this is able to happen before the pedal itself runs out of travel.

I appreciate you didn’t specify the upgrade components, but what you’re perhaps inadvertently finding here through your problems with bleeding is that the system doesn’t work fully as intended. What’s most worrying is it appears the bit of the system that doesn’t work properly is the safety failsafe. As you have such a mix and match of standard and non standard parts, trying to ensure everything works safely isn’t straightforward. There are several attributes at play here, and the standard system was carefully specified to ensure the required performance was achieved from all attributes.

Brake bias front to rear. Is this correct? Do the brakes operate in a manner that allows the car to remain stable under all braking scenarios (or at least those the original car was validated to)?

Pedal travel. Do the brakes apply at the correct point in the pedal travel? Is everything sized so you can achieve maximum braking before running out of pedal travel?

Piston travel. Can maximum braking on each axle be achieved whilst still retaining sufficient surplus piston travel?

Twin circuit failsafe. In the event of one circuit failing, is the other circuit still able to operate to provide full braking on that axle?

Brake fade. How frequently can you can you have a heavy braking event before you experience brake fade? Is this sufficiently frequent?

Brake performance. Is sufficient stopping power available on each axle? Has the change in spec actually improved anything? If so what is the trade off to achieve that, and more importantly has that trade off effected the systems ability to meet one of the other attribute requirements?

I don’t want to sound harsh but, unless you’re able to resolve all those questions, again I recommend either returning the system to standard, or taking it to someone that can validate the suitability of whatever components are specified.

[Tom W]

I should also add to the above list of attributes:

Total braking performance. Can you lock the brakes? Once you can lock the brakes, the system you have is already capable of stopping the car as quickly as possible. To improve on this, you’ll need tyres with more grip, or to shift the brake balance back towards the axle that still has grip. However, does shifting the brake balance here adversely affect the balance elsewhere?

Brake modulation and feel. How easy is it to feel when the brakes are reaching the point where they’re just about to lock? Is it easy to modulate back from here? This allows you to access maximum braking performance.

Pedal effort. Is that appropriate? Does the car stop ok but require undue effort, or are the brakes light and over sensitive?

[mtnjag]

Thank you for the feedback and suggestions. Have read through the referenced posts. Working with an experienced mechanic, will ensure I address the concerns and will report back results.

Any further comments, suggestions, info is welcome.

Layne

[mtnjag]

The article is very good but the pictures are two fuzzy to read the writing and the image is unclear like they are out of focus but I doubt this to be the case.
What do I need to do to make the pictures clear so I can read the writing?

[mgcjag]

Hi Layne...what artical are you refering to?....Steve

[mtnjag]

The link up the string that abowie (Andrew) included in his post.

[Tom W]

I imagine the pictures were hosted on photobucket. It used to be free to host photos on there. They’ve now introduced some sort of pay wall, so everything is fuzzy until you cough up some money.

[mgcjag]

I can see everything there nice and clear...I dont pay anything or have a photobucket account....Im useing an android Samsung tablet & Chrome....maybee try another device/browser....Steve

[mtnjag]

Was using an iMac and safari. Switch to Chrome on same but still blurry. No android or Microsoft devices. Do see the blurry issue on web if you don't pay, started last year. Guess they're missing you for some reason.

Anyway, back to subject.

[dxke38]

Just a point re master cylinders, several years ago I bought new ones for my series one from DMG and duly fitted them. I was very disappointed with the braking even though I had also fitted Coopercraft. It turned out that the new master cyls were 3/4" and not 5/8". This ruined the Hyd gearing, upon fitting 5/8" m/cyls all was right again.
Derek

[mtnjag]

Also the 3/4" don't have at the short 1" travel for the rear that the stock do. That appears to be an integral part of the failure mode. But the wilwoods have 4.8" piston area vs, the stock 2.125" fronts at 3.55" area as measured per side.

Not sure what you mean by "ruined the Hyd gearing"??

[dxke38]

Hi, gearing as in the size of master cyl piston in relation to the wheel cyl pistons. If you increase the size of the master cyls its as bad as reducing the size of wheel cyl pistons and increases the pressure needed at the pedal.
Derek

[mtnjag]

Yes but with increased slave cyl size existing MC piston and therefore pedal travel will be increased. Not saying the 3/4" is necessary at this point.

Are the Coopercraft caliper pistons a larger piston size than stock?