4 or 5 speed rear brake cylinders??

[TR7TIM]

Is it just me...........or did I read somewhere, that fitting four speed rear wheel cylinders will increase the brake pressure, therefore improving rear brakes?? I need to replace a leaking wheel cylinder, and would replace both at the same time.
Thanks Tim.

[dycecooper]

I was told this too, I have bought the cylinders but never fitted them

[prlee]

Would this make the rear prone to locking up?

Pete
1981 Carmelian Red 2 litre DHC.
Polybush, KYB shocks, electric fan, electric windows and electronic ignition roller bearing struts and anti-dive kit - otherwise standard
1979 Pageant Blue Spitfire 1500 - some restoration in progress.

[bmcecosse]

The pressure to the rears is limited by the PRV - if you want more rear braking, adjust the valve. But frankly it is set as it is to give a decent balance. I find the rear brakes (9" five speed car) get hot - which means they are doing a worthwhile job! Last thing you want is the rear wheels locking up on a wet road....

[dursleyman]

This is an old chestnut that keeps coming round so I actually made up a little page about wheel cylinders you can see here:-

https://docs.google.com/file/d/0Bz1MK9Y ... sp=sharing

The 4 speed and TR8 cylinders can be fitted to the normal 5 speed cars but you have to re-drill the back plates because the locating pin is in a different place. The pressure in the rears is limited by the PRV but because of the larger surface area those bigger diameter cylinders will give more braking force for that same pressure.

BUT the real question is why are you doing this? It will affect your front-rear balance and cause rear lockup unless its is balanced with better fronts to go with it. You will likely find yourself through the hedge backwards so it really needs thinking about.

Russ

1980 TR7 Sprint DHC
Dursley
UK

http://tr7russ.blogspot.co.uk/

[darrellw]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by dursleyman</i>

This is an old chestnut that keeps coming round so I actually made up a little page about wheel cylinders you can see here:-

https://docs.google.com/file/d/0Bz1MK9Y ... sp=sharing

<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

Hi Russ,

Just a minor nit, but under the picture of GWC1212, you say it was fit to "TR5 5spd and late autos", where I think you meant TR7.

-Darrell

Darrell Walker
66 TR4A IRS-SC CTC67956L
81 TR8 SATPZ458XBA406206
Vancouver, WA, USA

[busheytrader]

When I uprated my front brakes I also fitted 4 speed cylinders to the rear, as recommended by the guys at S&S. "A little extra braking effort at the back since there's a lot more at the front". I couldn't tell you how much extra braking effort these cylinders make since everything was changed at once. The MOT man is impressed each year.

It was a straight replacement but as Dursley said a tiny hole (2.5mm?) has to be drilled in the backplate to accomadate the locating pin. Simples....

The rear brakes have never locked up, even when I've locked the front up in the wet and that takes some doing.

I wouldn't have changed the rear cylinders if I hadn't uprated the front brakes first.............I found them pretty poor and they locked up under spirited driving in the dry all too readily.

Adam



TR7 V8 DHC Jaguar Solent Blue. 9.35cr Range Rover V8, Holley 390cfm, JWR Dual Port, 214 Cam, Lumention, Tubular Manifolds, S/S Single Pipe Exh, 3.08 Rear, 200lb Spax & PolyBushes all round, Anti- Dive, Strut-Top Roller Bearings, Capri Vented Discs & Calipers, Braided Hoses, 4 Speed Rear Cylinders, Uprated Master Cylinder & Servo, AT 14" 5 Spokes or Maestro Turbo 15" Alloys, Cruise Lights, S/S Heater Pipes, Replacement Fuel Tank. No Door Stickers. Mine since July 1986, V8 from 1991 courtesy of S&S V8 conversion and big brake kits.

[FI Spyder]

I concur. Unless you are updating your front brakes don't touch the rears. On a dry road on a curve I had to brake sharply to avoid running into a baby deer. The rears locked up (fronts didn't). The rear stepped out but as I was able to lift off before getting to the deer the rear tucked back in quite nicely. I was impressed with the car control as this was my first emergency maneuver. It was a case of rears locking up before the fronts. I have stock size tires, pads, shoes.

I have locked up all four wheels in a straight line in both wet and dry. True these where panic stops with not much (no) modulation.

If you have bigger (wider), high performance (sticky) tires you may not be able to lock up your brakes. Upgrading brakes should give you more modulation control but in most panic stops, there is no time for that and it's a reflexive reaction and bigger brakes would give no more braking as a locked brake is a locked brake.





- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

[dursleyman]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by darrellw</i>

Hi Russ,

Just a minor nit, but under the picture of GWC1212, you say it was fit to "TR5 5spd and late autos", where I think you meant TR7.

<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

Darrell, many thanks my typo well spotted - now corrected.

Russ

1980 TR7 Sprint DHC
Dursley
UK

http://tr7russ.blogspot.co.uk/

[Hasbeen]

Do be very careful of getting the thing to locking back brakes, I found it a bit hairy.

Move on now if you don't want to read about an old bloke telling you stories.

In 68 I drove the then new Holden Monaro 327 "Bathurst" model in the Bathurst 500 miles as it was then. I even ended up second.

The Monaro was one of the early cars to have duel braking systems, one for front & one for rear. However they made one big mistake. Not only did they divorce the systems, they also divorced the master cylinder reservoirs. The reservoir was partitioned down the center, leaving 2 very small sections.

Due to other circumstances my co driver had only given me a toilet break, & my pad change had been done about 20 laps early. The capacity of the reservoir for the front was only enough to follow up the pads to about 2/3Rd wear. With 13 laps to go, I was about to discover this fact rather graphically.

Approaching Skyline with no fluid left for the front brakes, I applied the pedal as usual, & the car leapt sideways. This was an instant change of direction, not just a bit of a slide. With no front brakes at all the backs locked solid instantly when I touched them.

I went over Skyline with the car sideways, with every bit of opposite lock it had applied. As the car slowed, this overcorrection suddenly bit, & the thing flicked back sideways the other way. By sheer good luck, this was just where the next corner went left, I got around that, & after a couple of fishtails, arrived at the Dipper slow enough to get round it.

Later some spectators told me about the course commentator raving about my brilliant save, which proves how much bullshit is called by said announcers.

I tried the brakes, very gingerly again at Forrest Elbow, scaring what ever was left to be scared out of me, when it locked solid, & I near lost it again. Just the slightest touch going down Conrod convinced me not to even think about it again.

I did 13 laps of Bathurst with no brakes. I was going back to third at top speed, & to second at just under 100 MPH. After nursing the thing through 108 laps of Bathurst, it was going to finish reasonably quickly, or blow up, I no longer cared very much.

Amazingly, given the treatment it did the next 13 laps, & did them only 3 seconds slower than my previous lap speeds. The brakes were pretty diabolical even when working at their best. The thing even got second.

After scrutineering they bleed the brakes & a Holden mechanic drove it back to the testing grounds in Melbourne, about 700 miles without trouble. Amazingly when stripped the thing showed no sign of the thrashing I had given it. Tough things those 60s Chevy V8s

Hasbeen

[fastman]

I had done this mod on my TR7 FHC. It also had the proportioning valve replaced with a 50/50 unit from "The Wedge Shop". I'm running 205/50/15s with very good tires, although they are now 4 or 5 seasons old. I have not experienced any braking issues. The car stops straight and I have never locked the back brakes. I'm also not sure I can tell the difference with the mod. The proportioning valve did have a positive difference, as the braking seems to have the car stopping with more control. This of course could just be confidence.

[bmcecosse]

The 50/50 unit may have negated the larger bore cylinder benefit! Does anyone know what the standard ratio is on the PRV ? On my old Mini - I threw the hateful PRV away and fitted a pressure limiting valve on the run to the rear brakes. I could then fine tune the pressure by adjusting the spring in the valve. It was much better on the Mini - which had uprated discs (larger discs and pads than a TR7...) fitted. But I find my TR7 just fine as it is, and have no intention of changing anything!

[fastman]

Was the ratio something like 70:30 F:R? Or did I just make that up?

[bmcecosse]

When my Mini came to me - the PRV had been connected wrong way round - heavy pressure to the rears, and hardly any to the front discs...now THAT was interesting with a full 360 in traffic on the way home.....[:0]

[dursleyman]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by bmcecosse</i>

When my Mini came to me - the PRV had been connected wrong way round - heavy pressure to the rears, and hardly any to the front discs...now THAT was interesting with a full 360 in traffic on the way home.....[:0]

<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

Its a case of poor design when something important like that can be connected the wrong way round. We were always taught that you should design safety critical stuff so its impossible to connect it back to front.
Wasn't there a case years ago where a one-way valve in an aircraft fuel system got fitted in reverse and caused an accident?

Russ

1980 TR7 Sprint DHC
Dursley
UK

http://tr7russ.blogspot.co.uk/