Rear Disc Conversion

[steveg]

Installed proportioning valve inline between the 5-way and rear brake line:

As was suggested we began by doing a "jackstand" test. The "dreaded assistant" put slight increasing pressure on the brake pedal as we turned a front and rear wheel, adjusting the prop valve to find the lightest-pressure-position where the front wheel locks and the rear wheel has braking drag but can still be moved.

Then did the road test and found the car stopped straight with no fishtailing on the panic stop.

It appears the proportioning valve, by decreasing rear braking, increases the pedal pressure required, though not by a large amount.

 

[Lin]

Hi Steve,
Which valve did you use and from which vendor?
Thanks,
Lin

 

[steveg]

For the valve:
Allstar ALL48025 Brake Proportioning Valve - on Amazon.

It appears to be identical to the Wilwood but a couple of bucks less.

Finished my photo essay on the project:
https://www.pbase.com/stevegerow/healeyreardiscs&page=1

Next project will be to install the Fiat 124 dual remote master cylinder and the revised brake lines.

 

[RAC68]

Hi Steve,

First, as expected your implementation is quality. I hope you don’t mind my asking a few questions. Are you still using the original Girling front calipers? How do the back rotors and calipers relate to the front Healey calipers and disks in size and pressure development? I am wondering as the calipers and disks in your pictures seem quite large.

Since the wheel/tire combination on the rears is identical to the front wheel/tire combination, I can see using a proportioning valve to reduce rear caliper pressure to eliminate lock-up if the rear setup is equal or larger than the front. However, there are two conditions that I am not sure of. In the original disk/drum setup, the application of the rear self-actuating drum brakes is delayed by the counter pressure applied by the brake return springs. When this reverse pressure is overcome, the stopping power of the Healey’s large rear drums builds much faster after a slow start and passes the front disks at higher fluid pressures.

With 4-wheel disks, rear and front calipers will apply stopping power without rear delay. Since the front disks will need to stop more weight then the rears, especially under severe stops when an additional 10% or rear weight transfers to the fronts, far less stopping force needs to be applied or the reduced weight/traction will cause a rear wheel lockup and eliminate the rear brakes as an effective stopping force. Since the proportioning valve was used to reduce fluid force at the rear disks, was it adjusted for severe stops or light braking as the latter will not require as much of a reduction in force?

Additionally, with drums, to move the initial application of rear drum brakes to coincide with the front disks, a metering valve is usually used but, as far as I know, this valve does not seem to be required or present on the Healey. Have you found anything in the Healey that seems to perform as would a metering valve or are the rear drum brakes applied after the front disks If the rear drums are applied after the front disks, is this condition necessary to replicate with the with the implementation of rear disks?

Sorry to be asking all these questions but my understandings are only hypothetically at present and, as I value stopping as more critical a function than going, I appreciate learning from your experiences in preparation to some day follow your lead.

Thanks and all the best,
Ray (64BJ8P1)

 

[healeyblue]

Steve
Great job. You should merge some of your threads together to help answer some of the questions like Ray has
Here are some that came to my mind.
https://www.britishcarforum.com/bcf...ow-complete-photo-essay&highlight=toyota+disc

https://www.britishcarforum.com/bcf...n-Disc-Brake-conversion&highlight=toyota+disc

 

[steveg]

Don't know how to do that, so will just answer Ray's questions.

 

[steveg]

Ray,
They're Toyota 4-piston fronts on late BJ8 swivel axles with the thicker late BJ8 discs. See:
https://www.pbase.com/stevegerow/healey_discs

The stock disc drum setup has either smaller or larger (forget which) rear wheel cylinders than the all-drum cars. IMHO this provides a proportioning effect.
I don't know the answers to the drum technical questions but the 4-wheel disc setup stops in a shorter distance than the disc-drum setup did. The proportioning valve is necessary to tune the rear brakes to the fronts. The fronts are left at full strength and the rears are dialed back until the rear doesn't skid any more during a panic stop.

I adjusted the proportioning valve for severe stops. The car stopped straight enough in ordinary driving but was pretty exciting when I did a panic stop pre-proportioning valve.
If the front calipers are less powerful, one would have to dial the rears back even more, resulting in less braking power over all.
The Richard Mayor (Boyracer) discs are 11.25"D vs 11" on the Denis Welch and Cape International. The difference shows.

 

[RAC68]

Thanks Steve.

I must have missed the 4-pot front calipers. So, with the dual channel MC, new line layout and rear disks, you will have completely changed your Healey’s braking system. Wow, that is impressive.

I hadn’t considered the Healey needed increased stopping power unless you are racing but like the idea of replacing the drums with disks for a number of reasons including loss of unsprung weight and drum balancing issues. Do you know if anyone has retained the front calipers and replaced the drums with smaller rear calipers and disks?

Thanks again,
Ray (64BJ8P1)

 

[steveg]

Have heard of someone using '90s T-bird rear calipers. I like the Jag setup because it looks similar to the disc setups of period cars, such as wire-wheel 289 cobras and '60s Ferarris. Modern single-piston calipers are gauche. Wilwood makes a caliper with parking brake - maybe that's the one Cape or AHSpares (maybe) sells. All too modern and anodized.

 

[HealeyPassion]

Steve, I will be interested in your efforts with the proportioning valve. I've just installed one on mine... but I'm a long way from actually testing it. I'm changing everything.... as any self-respecting obsessive-compulsive person would be expected to do.... Wilwood dual master cylinder, prop. valve, residual pressure valve, triple remote fluid reservoirs, Fosseway 4 piston cals., etc. ..