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Front brake pipe question

Jasper - 28/12/02 at 06:58 PM

I'm using a Sierra master cylinder. I was gonna block off one of the two front pipe connections, and then split the single pipe half way around the front system (I've already begun to do all this)

But after seeing Bob's set-up of seperate pipes to each of the front calipers I'm now wondering which way is better - or does it - as I suspect - make no difference at all.

theconrodkid - 28/12/02 at 08:56 PM

best way would be to have seperate pipes to the front calipers and a single to the rear T peice,remember front outlets are nearest the pedal

bob - 28/12/02 at 09:42 PM

quote:
Originally posted by theconrodkid
best way would be to have seperate pipes to the front calipers and a single to the rear T peice,remember front outlets are nearest the pedal

No No No No No please dont tell me front outlets are nearest the pedal

David Jenkins - 28/12/02 at 09:54 PM

I think it depends on the master cylinder - on mine, I have 2 outlets on the front (i.e. furthest from the pedal) that go to the front brakes. The single outlet nearest the pedal goes to the back brakes.

bob - 28/12/02 at 10:06 PM

Thanks Dave,spot on.
just checked my notes from dismantling the sierra to confirm phew,was woried there for a bit.

Jasper - 29/12/02 at 09:35 AM

Cheers chaps......

theconrodkid - 29/12/02 at 09:49 AM

there are lots of diferent cylinders 3 or 4 port some split diagonaly but most are the way i described,works well on mine

bob - 29/12/02 at 10:05 AM

I can confirm they work well on johns car,as long as they work i suppose.
Have read somewhere of people swapping them around to change pressure.

[Edited on 29/12/02 by bob]

jollygreengiant - 29/12/02 at 11:32 AM

The only word of wisdom about ford Master Cylinders that I have to give is, DON'T EVER remove the plastic tag around the cylinder, cos there are so many variations that ford did, that the Numbers on them are critical for getting the right replacement, should you ever need to replace.
And the Numbers are NOT stamped on the Bodies

Stu16v - 29/12/02 at 01:23 PM

The front brake line ports are usually-but not always the furthest AWAY from the pedal. Idea being that the rear brakes get activated very slightly before the fronts when brake is applied (motorcyclists will know about this theory...)

Dick Axtell - 30/12/02 at 08:34 AM

quote:
Originally posted by Stu16v
Idea being that the rear brakes get activated very slightly before the fronts when brake is applied...

The reason for applying pressure to the rear (drum) brakes first, was to overcome the fitted loads of the shoe return springs, so that front & rears were applied as near to simultaneously as possible.

Western auto practice - always connect the fronts to the larger mcyl chamber (which you correctly identify as that furthest from the brake pedal). This applies to the older type 60/40 mcyl used in F/R split systems. X-split systems, generally used on more modern fwd vehicles, are pressurised with a 50/50 mcyl (not recommended for Locosts).

Stu16v - 30/12/02 at 01:10 PM

Ok so I am confused now, the Sierra (RWD) amongst others have a front/rear split and a 50/50 M/cylinder. And admittedly, I havent had many master cylinders apart (because I always replace them rather than f*ck about trying to re-seal them) but I dont ever recall seeing one with different sized pistons in......

Dick Axtell - 31/12/02 at 01:16 PM

Hi Stu,

NO! The Sierra mcyl (p/n 74066315 - I think!) is definitely NOT 50/50. It is nearer to 62/38, but 60/40 will suffice.

Just to clarify things :-
1: Assume that the mcyl is mounted with its flange towards the pedal, & its closed end towards the front of the vehicle (ie in same direction as per donor).
2: The piston nearest the pedal is the primary, & that nearer to the front of the vehicle is the secondary.
3: The primary piston will always travel more than the secondary, by compressing the primary chamber. Theoretically, this is a simultaneous cut-off mcyl (ie both chambers together).
4: In order to prevent piston seal damage (nibbling) by the cut-off port, the rear brakes are always connected to the primary chamber. This ensures that when the shoe return springs cause the primary chamber to rise, the primary piston seal has stroked safely away from the cut-off port.

It might be useful if I scanned in some diags to help illustrate the above points, but if you still want more clarification, let me know.

jollygreengiant - 31/12/02 at 01:22 PM

Minor differences are in, bore size, piston and seal constuction, stroke length and front to rear effort compensation by virtue of the floating piston and valve assembly therein.

Some modern designs of master cylinder are sooooo sensitive that ('90' on vauxalls come to mind) that if you push ANY fluid the wrong way (push a piston back to change a brake pad) you end up with a reversed seal in the Master cylinder and end up buying a new one.

Enjoy. Wish you all a Happy New year.

Stu16v - 31/12/02 at 06:52 PM

Dick, thanks for your reply. I would appreciate any pics or info you could scan for me because I am having difficulty getting my bonce around the theory.....If there is no air in the M/cyl, the pistons must travel as one as fluid cannot be compressed right? And if the bore size of the cyl is same end to end the same amount of of fluid must be displaced for a given pedal stroke. It does look like I need educating....

Cheers, Stu

Duncan - 31/12/02 at 07:56 PM

I thought I would add my comments for what they are worth.

Surely the fluid in the hydraulic system is constantly travelling in both directions when the brakes are applied and then pressure is removed from the pedal

When the pedal is pressed, fluid is displaced from the master cylinder, down through the brake pipes and hoses and into the brake cylinders and calipers, thus allowing the brakes to be applied

When pressure is removed from the pedal, the return springs on brake shoes and the fluid seals in the calipers cause the pistons to retract, pushing the fluid back in the opposite direction, up through the pipes towards the master cylinder

(I know it doesn't travel all the way, but hopefully you understand what I mean)

On another note, the only car I have seen with a different size primary and secondary piston is a mini. When a replacement master cylinder was supplied it came with new brake pipes that had to be fitted otherwise the rear wheels would lock up when you applied the brakes

Jasper - 31/12/02 at 08:11 PM

That's what I love about the forum - you ask a simple unassuming question, and a get a wealth of fascinating info in return!!!

BTW - Yes it is 8.15pm on new years eve, that's what having a family does for you!!

Dick Axtell - 2/1/03 at 08:47 AM

quote:
Originally posted by Stu16v
I would appreciate any pics or info you could scan for me because I am having difficulty getting my bonce around the theory.....

Stu, I forgot to mention the assumption that the mcyl would be fully hydraulically bled (so no pneumatic compression).
Can get to a scanner when the office opens next week; am delving thru my files to find the appropriate pics.
Watch this space.

Stu16v - 3/1/03 at 01:58 AM

I have dug a few manuals, techy books etc out and at the moment at least, they all seem to back up my original understanding of how a dual circuit M/cyl works. Interestingly, an old manual for the Mk2 Escort shows the the front/rear pipes on one way for LHD, and the other way for RHD!
Assuming the primary and secondary pistons/ports are the same bore size (which the majority of cars are AFAIK) the hydraulic pressure supplied by the cylinder is the same. The primary piston cannot actually compress any more if there is no air in the system, what does happen is that fluid is squeezed into the primary circuit. But the point is reached when the primary piston drives the secondary piston foward (via the fluid in the primary chamber) and subsequently begins to activate the front circuit. There maybe slightly more fluid available to the rear circuit by virtue of the fact that the primary chamber collapses in size, BUT actual pressure in the pipes will be equal. There is a front to rear brake bias, but this is usually sorted by the different hydraulic and mechanical design of the front and rear brake components, not forgetting LSVs and inertia valves which will alter the static theoretical ratio again. Otherwise most other cars are to be found with pressure limiting valves in the rear circuit.
But please, if you can supply info to educate me otherwise, please do, because after all, every day is a school day....

[Edited on 3/1/03 by Stu16v]

paulf - 4/1/03 at 08:19 PM

I also wondered how the front and rear pistons could displace different amounts of fluid, but after thinking for a while and loking at a diagram i realised that altough the bore of the cylinder is the same the piston area is not.This is due to the fact that the piston nearest the pedal is joined to the second piston by a pushrod that takes up some cylinder volume , the effective area is therefore less than the front piston that is full bore size and therfore a larger area.
Paul

quote:
Originally posted by Stu16v
I have dug a few manuals, techy books etc out and at the moment at least, they all seem to back up my original understanding of how a dual circuit M/cyl works. Interestingly, an old manual for the Mk2 Escort shows the the front/rear pipes on one way for LHD, and the other way for RHD!
Assuming the primary and secondary pistons/ports are the same bore size (which the majority of cars are AFAIK) the hydraulic pressure supplied by the cylinder is the same. The primary piston cannot actually compress any more if there is no air in the system, what does happen is that fluid is squeezed into the primary circuit. But the point is reached when the primary piston drives the secondary piston foward (via the fluid in the primary chamber) and subsequently begins to activate the front circuit. There maybe slightly more fluid available to the rear circuit by virtue of the fact that the primary chamber collapses in size, BUT actual pressure in the pipes will be equal. There is a front to rear brake bias, but this is usually sorted by the different hydraulic and mechanical design of the front and rear brake components, not forgetting LSVs and inertia valves which will alter the static theoretical ratio again. Otherwise most other cars are to be found with pressure limiting valves in the rear circuit.
But please, if you can supply info to educate me otherwise, please do, because after all, every day is a school day....

[Edited on 3/1/03 by Stu16v]

Stu16v - 5/1/03 at 02:45 PM

quote:

I also wondered how the front and rear pistons could displace different amounts of fluid, but after thinking for a while and loking at a diagram i realised that altough the bore of the cylinder is the same the piston area is not.This is due to the fact that the piston nearest the pedal is joined to the second piston by a pushrod that takes up some cylinder volume , the effective area is therefore less than the front piston that is full bore size and therfore a larger area.

Hmmmm. In all the exploded piccys I have sourced so far, the piston areas for both chambers have been pretty much identical. If your theory was the case, then that would mean the primary port would have the mechanical advantage i.e imagine using two different sized M/cyls as in a bias bar system, the smaller one will allow greater force to be applied to an axle, but for greater pedal travel.

[Edited on 5/1/03 by Stu16v]

paulf - 5/1/03 at 10:10 PM

Yes this is true but the length of travel of the primary piston is limited by the stroke of the secondary piston , therefore limiting the pressure developed in the primary circuit.If the secondary circuit was disconnected then your thery would apply but the secondary travel limits the pressure in the primary circuit.
Paul.

Hmmmm. In all the exploded piccys I have sourced so far, the piston areas for both chambers have been pretty much identical. If your theory was the case, then that would mean the primary port would have the mechanical advantage i.e imagine using two different sized M/cyls as in a bias bar system, the smaller one will allow greater force to be applied to an axle, but for greater pedal travel.

Dick Axtell - 6/1/03 at 04:33 PM

quote:
Originally posted by Dick Axtell
Can get to a scanner when the office opens next week; am delving thru my files to find the appropriate pics.

Stu - Re pics; have run into the fairly major problem of copyright restrictions. Looks like I'll have to concoct my own sketches. Still on the case.

Dick

Stu16v - 6/1/03 at 08:11 PM

quote:

Yes this is true but the length of travel of the primary piston is limited by the stroke of the secondary piston , therefore limiting the pressure developed in the primary circuit.If the secondary circuit was disconnected then your thery would apply but the secondary travel limits the pressure in the primary circuit.

The primary piston is NOT limited by the stroke of the secondary piston. Both circuits work completely independantly of each other, even though they are operated by the one brake pedal. The cylinder basically works by the primary piston acting upon the secondary piston via the slug of brake fluid between the two primary piston seals, thereby giving equal brake pressure to both circuits. In the event of a primary circuit failiure, the primary piston area collapses, so the two seals effectively meet and goes on to activate the secondary piston. If the second circuit fails, the secondary piston bottoms out at the end of the cylinder giving the primary circuit something to pressurize against.
P.s. During my 'investigation' I have found some more reasons why it is important to ensure that the M/cyl is plumbed up correctly. Some cylinders have a residual pressure valve built in to keep a very small amount of pressure in the hydraulic system. This prevents seals leaking, and ingress of dirt. This usually only applies to the primary circuit only, and then only if the system is front/rear split. Also a few M/cyls have a bigger reservoir for the secondary circuit, to ensure that when the pads in the calipers wear, there is plenty enough fluid in the system to compensate.

Cheers, Stu.

[Edited on 6/1/03 by Stu16v]

jollygreengiant - 7/1/03 at 12:03 AM

Wisdom is not given it is earned. Give people enough direction & they usually reach the right conclusions.

That what I like about this forum. People learn lots.

Enjoy.

Dick Axtell - 10/1/03 at 10:14 AM

quote:
Originally posted by paulf
loking at a diagram i realised that altough the bore of the cylinder is the same the piston area is not.This is due to the fact that the piston nearest the pedal is joined to the second piston by a pushrod.
quote:

Definitely NOT. That "pushrod" is either a stroke limiter, or a means of carrying the primary return spring. It merely abuts the rear face of the secondary piston, and is not sealed to it. Therefore, the primary piston area ia the full diametra area.

Still working on these diagrams - always necessary for such explanations.

Dick Axtell - 10/1/03 at 10:19 AM
quote:
Originally posted by jollygreengiant
The only word of wisdom about ford Master Cylinders that I have to give is, DON'T EVER remove the plastic tag around the cylinder, cos there are so many variations that ford did, that the Numbers on them are critical for getting the right replacement, should you ever need to replace.
And the Numbers are NOT stamped on the Bodies

Hi JGG, Couldn't agree more. Any ID is better than none. However, any number stamped on a mcyl body will probably be a casting part number, and will be relative to the mcyl supplier, not FoMoCo.