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volvo 240 brake junction/brake line delete with 91 style

jjona5

Member
Joined
Nov 15, 2017
Location
Houston
I have swapped all of my non abs cars to the 91 style brake junction as they don't leak. That said, my v8 car is heading to a brake upgrade and I have that weird 91 style. Does the delete still run the same way as is shown on https://www.240turbo.com/volvo240bigbrakes.html

Also I'm not completely sure how just plugging those two lines up would fix things. With the 91 style should I move both rear lines to one half of the junction with one side of the master feeding it?

I also plan on installing a line lock at some point so I'd love to get it so one port off of the master just does the front brakes and the other just does the rear.

This is the style block I have now
3540084-1991-240.jpg


I read this thread but didnt get what I wanted out of it. I wish I had a spare 91 style block so I can see how the circuits work. http://forums.tbforums.com/showthread.php?t=71515
 
There is nothing complicated about the junction block. Each end is entirely separate. It's simply two distribution manifolds, with one input and three outputs each. The older one has a valve in the middle to identify a pressure imbalance caused by a leak. The two halves remain separate.

If you take a while to look at how it's all plumbed, you'll see it's not that complicated really. Each half of the master (and junction block) feeds two pistons in each front caliper and one rear caliper. Once you've got rid of the two hose front calipers (as you are most likely to do when upgrading, there aren't many other cars out there with the same setup), then it becomes a fairly normal diagonally split braking system.

If you then want to rearrange it to make it a front/rear system you could then move some of the lines around on the junction block. Or better, just replumb the car from scratch.
 
When dealing with a non ABS car, your master cylinder is stepped. Two different size pistons. I' have asked before and have never found a good answer if this will create issues. Obviously it would not with a front/rear setup. But I have trouble understanding how one line from a small piston can go to one rear caliper and one line from a large piston can go to the other rear caliper. Maybe not if they weren't coming from a chamber that feeds a front caliper. Dont know if the answer is in that somehow.
Dave
 
Wouldn't the pressures still be the same with one line in each circuit plugged?

This is why I question this and say probably not. If two holes are plugged with a stepped master, then the two lines going to the rear calipers are coming from two different sized pistons. So one rear caliper potentially gets more pressure that the other. Unless of course the front caliper for each line somehow dampens it.
 
I don't claim to understand how it works but that article says the pressure in both circuits is equal with an intact system. Plugging one line in each circuit should have no effect on the pressure since the equalization is done in the master. Rerouting the lines so that the system is split front/rear should also have no effect on the pressure.
 
I don't claim to understand how it works but that article says the pressure in both circuits is equal with an intact system. Plugging one line in each circuit should have no effect on the pressure since the equalization is done in the master. Rerouting the lines so that the system is split front/rear should also have no effect on the pressure.

I'll take that as the best answer so far.
Dave
 
Hmm. It is weird that rear brakes are on both circuits(left on one right on the other)...that is what was throwing me. So I guess if I was going to make a line lock I would need to put both of the rear brakes on one circuit and both of the front brakes on the other and then plug the extra holes. Then take it to a secluded parking lot and make some braking turns and see if I need any bias work.
 
When dealing with a non ABS car, your master cylinder is stepped. Two different size pistons. I' have asked before and have never found a good answer if this will create issues. Obviously it would not with a front/rear setup. But I have trouble understanding how one line from a small piston can go to one rear caliper and one line from a large piston can go to the other rear caliper. Maybe not if they weren't coming from a chamber that feeds a front caliper. Dont know if the answer is in that somehow.
Dave

The rear-most piston creates the pressure, which is transferred through the front piston. The only time you could see a difference is if the front piston is acted on directly, in which case the rear piston is dead.
I thought about that way to much, then did a big forehead slap, and put gauges on to prove it.
 
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The rear-most piston creates the pressure, which is transferred through the front piston. The only time you could see a difference is if the front piston is acted on directly, in which case the rear piston is dead. The result is a less powerful piston with a better feel (compared to a small bore after the pedal has dropped due to failure).
I thought about that way to much, then did a big forehead slap, and put gauges on to prove it.

So then are the two pistons not connected mechanically? Only connected by movement of fluid? That would make sense to how pressures can be equal.
Dave
 
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The rear-most piston creates the pressure, which is transferred through the front piston. The only time you could see a difference is if the front piston is acted on directly, in which case the rear piston is dead. The result is a less powerful piston with a better feel (compared to a small bore after the pedal has dropped due to failure).
I thought about that way to much, then did a big forehead slap, and put gauges on to prove it.

Now is that because the front calipers are acting as the equalizer? Since the front and rear line go to both the front calipers?
 
No, the equalization is done in the master.

The pressure equalization necessary to obtain the same pressure in
both circuits is achieved in the stepped bore master cylinder
with a secondary piston, which floats within certain limits.
 
So then are the two pistons not connected mechanically? Only connected by movement of fluid? That would make sense to how pressures can be equal.
Dave

All 2-piston masters work that way. Under normal operation the fluid separating the 2 pistons pushes the front piston, which pushes the fluid into that circuit.
If the rear system fails, loses fluid, has bubbles, etc. and the 2 pistons touch, then the pedal pushrod is directly acting on the front piston. There's a nose on many rear pistons for that.
 
*edit to fix moderately drunk post:

Hypothetically speaking, if the rear piston is 1.5 times the area of the front, then (only) when they touch, the front displaces .66x the volume of fluid per pedal distance (in the piston, it divides when it hits the lines), at 1.5x the pressure of the properly operating tandem system (per front or rear circuit, the same in each line)

When the front fails and it bottoms out, the rear piston area dictates the pressure out compared to pedal force, same as when the system operates normally.
 
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hmm so then I guess the easiest way for me to get what I want is to put both rear brake lines on one circuit and both front brake lines on another and then cap off the extras. Then put a line lock on the line coming off the master.

Thanks for all the help.
 
Since I?m also about to replumb my car as a front/rear system, I thought I?d better find out exactly how the Volvo master cylinder works. That SAE paper hiperfauto refers to above explains it all.

The Volvo 240 stepped bore master is very nearly the same as a normal tandem master, but with one key change (apart from the stepped bore, okay that?s two changes), an additional ?spacing sleeve? between the two pistons that prevents the secondary piston from moving too far away from the primary piston. The spacing sleeve is mechanically attached to both pistons, but with some movement allowed. This was introduced to prevent the sudden increase in pedal travel that would occur if a circuit were to fail in a standard tandem master cylinder system. You will also notice that the area of the large piston is exactly twice that of the small piston. This is important.



During normal use the primary piston (1) (22.2mm, nearest to the pedal) generates all of the pressure. The secondary piston (2) ?floats? just as in a normal tandem system with equal pistons. The floating piston equalises the pressure in both circuits (mostly, see aside 1). Since the 240 uses two identical circuits made up of a pair of pistons in each front calliper and one rear calliper, the amount of fluid displaced is the same in each. Piston 2 floats within the bounds allowed by the spacing sleeve. Everything works as normal.

If the primary circuit fails, that piston (1), unhindered by hydraulic fluid, moves forward and pushes against the secondary piston (2) via the spacer sleeve. This is fairly normal for a tandem master, except for the spacing sleeve. Pedal travel increases slightly and the pressure in the remaining good circuit is twice that of normal as the secondary piston is half the area of the normal working piston.

If the secondary circuit fails, the secondary piston is no longer floating between two chambers of hydraulic fluid, but it cannot move all the way to the end of the cylinder (as would happen in a normal tandem master cylinder) because it is restrained by the spacing sleeve. Because the primary piston is pushing twice as much fluid as the smaller piston 2 which is moving with it, the resulting fluid displaced is the amount displaced by the large piston (1) minus the amount displaced by the small piston (2). Since piston 1 is twice the area of piston 2 the amount displaced is the same as for just one of the smaller pistons, with the same result as for the previous failure case, the pedal travel increases slightly and the pressure in the remaining circuit is twice as much as normal.

Normal behavior depends on the volume of the two circuits being the same to allow the spacing sleeve to restrain the relative movement of the pistons within an acceptable range. There is a note in the SAE article that this type of master is okay on a diagonally split system (which the 240 triangular setup is, sort of), but would not work on a front/rear split system as it would be unlikely that the volume displaced in the front and rear circuits would be the same. Calliper piston retraction also has a part to play in this volume, so even if the callipers were the same volume, the actual volume displaced by the pistons retracting in two different types of calliper would be unlikely to be the same.

So what does this mean for rearranging the Volvo 240 system for a front/rear split?
Don?t. The stepped master cylinder is designed to work with two circuits which displace exactly the same volume of fluid. A front/rear split generally will not do that. It would probably work, but would behave unpredictably as it would be functioning as if one circuit had failed and therefore increase the pressure in the other circuit.

The stepped master cylinder can safely be replumbed to a standard diagonal split with alternative front callipers that have a single line, as thousands of people have done before, but if going for a front/rear split I would change the master cylinder to a normal non-stepped version (also see aside 2 below).

I?m guessing that Volvo changed the master cylinder to a normal non-stepped one for ABS models, which are a front/rear split? But we didn?t get ABS on many 240?s in the UK so I?ve never seen one.

Aside 1
There is a note in the SAE article that due to friction and the spring acting on the secondary piston there is a slight difference in pressure in each circuit and it is therefore important to minimise king-pin offset to prevent pulling to one side. It doesn?t quantify this difference, but I would imagine it is very small, especially with high pedal forces which will overcome the tiny spring in the master cylinder many times over.

Aside 2
Fast-fill stepped bore master cylinders are very different to the 240 stepped bore master cylinder. Fast-fill cylinders use one large piston to move a lot of fluid quickly to take up clearances in the brakes and then two smaller pistons to actually apply pressure and balance it between two circuits. So 3 pistons in total. The smaller piston(s) apply pressure and are the diameter that should be used for calculations. This type of master cylinder can be used for front/rear split systems.
 
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