DISC BRAKES & RESIDUAL PRESSURE VALVE

Post by **ronwilson** » Sun Jun 11, 2006 12:08 am

Not getting brake pedal prerssure in spite of numerous positive and back pressure bleeds. No leaks evident in the system, at the m/c or at the wheel cylinders, yet pedal goes to the floor & takes about 2.5 pumps to get pressure/ brake lights. Last year similar problem cured by increasing volume being pumped. System (Marina front discs/Ford Granada rear discs fed from MG dual cyclinder m/c with remote reservoir and Servo) has worked well for the past year; only developed problem after m/c cover was altered 2nd time to protect (from clumsy feet)the feed from remote reservoir. Hot rod friend suggested system might benefit from inserting a 2psi Residual Pressure Valve into the flow line as US rodders do with a m/c that sits lower than the wheel brake cylinders (as Morris does).

I've gone back thru some earlier posts on Disc brake threads but didn't find the one (Ray's ? I think) from last year that when installing disc brakes the m/c had to be modified to remove a cap on the m/c piston to allow the pads to back off from the disc. This Residual Pressure Valve used by Rodders would seem to be just the opposite of that advice. Confused-but appreciate I may be missing something that others might grasp. Any help ? Thanks, Ron

Post by **Innovator** » Sun Jun 11, 2006 6:02 am

Having the mc`s lower than the calipers has never caused me a problem with any of my cars.

I would also stay away from a residual pressure valve. It may fix your problem but I do not think it is the real cause.

You say it has only happened since you altered the mc cover. Try to look at this and suss out what may be different.

When you do get pressure is the pedal good? What I am thinking that is the actual amount of movement you get at the pads is very small. I worked out that if I had full master cyclinder travel it would only move the pads 0.008" (8 thousands on an inch). Now this isnt a problem with rigid caliper, true discs and no play in the bearings, but if the pads are getting pushed back in......

John

Post by **bmcecosse** » Sun Jun 11, 2006 9:34 am

But they worked before - so it must be something that's been done recently.

Post by **ronwilson** » Sun Jun 11, 2006 1:16 pm

Thanks BM and John,

We're relooking at the connections at the m/c, first T piece and the line(s) bx. Have found some resistance on one of the parallel lines from the m/c near the T piece. ( MG dual 7/8 has two input pistons and two line exits-we've hooked both pistons up to solely work the brake system.) Now in process of replacing suspect pipe section.

John, Will take a look at pad travel on depression.

Post by **ronwilson** » Sun Jun 11, 2006 3:42 pm

Update: Replaced suspect section of pipe and rebled the system. The Servo was and still is bypassed. No improvement.

Will have to relook at the pad travel, m/c and the wheel cylinders.

Most skilled technician now off to barbie so work suspended till tomorrow. Thanks again BM & John for comments. Ron

Post by **rayofleamington** » Sun Jun 11, 2006 5:54 pm

sounds like a prize pain!
You've done all the sensible things - bleeding and rebleeding checking for suspect lines etc...
In this case it's far from simple to differentiate between something that you've introduced or something that happened at the same time by concidence and you'll just have to go along piece by piece

Last year similar problem cured by increasing volume being pumped.

Do you mean that a year ago a similar problem developed by itself or that it was always like that and improved using the bigger flow m/c?

One possiblity for problems is brake flexi hoses - when they get old they can become much more compliant and you'll loose flow just to pressurise the hoses. I doubt however that you can loose 2.5 pumps of flow into the hoses!

When you say the problem came when modifying the m/c cap - I wonder if you have also tried swapping back to hgow it was to see if the problem is better or worse (or probably the same)?

When things start to get desperate with the brakes, you can see where the problem arises by isolating parts of the circuit. Unfortunately this needs time, determintion and a lot of patience.

Basically block off the m/c output with a bleed screw (or a bleed screw on the end of a short pipe if that's the only way to hook it up) and with that, the m/c should lock up instantly (nearly no piston travel)
Then connect up just the rears and you'll get a bit more pedal travel but if you get a lot this indicates a problem.
Same again with the fronts.
If the m/c on its own is good, I expect you'll find the problem in either the front or rear circuit 'somewhere'.
It may be easier to start with the full system then block out the front or rear, but it's swings and roundabouts as you may do all that work and both will look problematic if the problem is poor flow from the m/c itself.

Post by **RobThomas** » Sun Jun 11, 2006 7:34 pm

Just a thought but in the Midget the cylinder sits up at an angle with the pipes uppermost. Might there be air trapped in there?
I'm assuming you have the type that was used before the mid 60s?
Again, thinking out loud....have you ensured that the feed from the reservoir is large enough so that there isn't any sort of vacuum created by the large pistons being unable to suck fluid down into the cylinder body? Difficult one!

Post by **ronwilson** » Sun Jun 11, 2006 11:41 pm

rayofleamington wrote: sounds like a prize pain!
Last year similar problem cured by increasing volume being pumped.

Problem first arose during rebuild & new dual m/c (plus tries with several 13/16s) was solution.

New flexi-hoses were used in rebuild so only one year old now, but will add check of those to our list.

We went back to earlier cap-no difference in pedal.

Blocked off m/c and pedal locked up pronto.

As you suggest, will go thru sequence of isolating front & rears next. Removed & stripped the m/c 2 days ago, seals looked OK, but could be worth a relook at its fittings & check its flow again.

Thanks for advice. Will get started tomorrow.

Post by **ronwilson** » Sun Jun 11, 2006 11:49 pm

Rob, will relook at a Midget/Sprite setup. Think there's one down at local garage. Thanks, Ron

With our setup there's lots of ups & downs on the pipework. Didn't bother the system before, but if all else fails might have to change the runs as last resort. Gulp.

Post by **Willie** » Mon Jun 12, 2006 1:50 pm

If I read it correctly your problem arose after altering the 'M/C cover to
protect a brake pipe from feet' I suppose the alteration hasn't meant that the
brake pedal cannot return to its original height which would mean less volume
pumped when operated?

Post by **ronwilson** » Mon Jun 12, 2006 4:35 pm

No Willie, no affect on brake pedal travel. We created a new cover plate in brass using oe cover for dimensions. The MG Midget dual cylinder m/c has a central top filler cap. We welded a truck tyre valve with a lower profile into a brass plate and with new cork gasket fitted this into the chassis rail where the oe 13/16 m/c sat. The MG m/c fixes in the chassis same as Morris oe. The tyre valve allows a fixing for the rubber hose to fill from the remote reservoir and its low profile allows underlay to fit either side and bring it flush with carpet. Hence, protect from clumsy feet.

Post by **rayofleamington** » Mon Jun 12, 2006 4:59 pm

presumably this truck tyre valve had all the valve components removed!? (sorry for the daft question!!)

Post by **ronwilson** » Mon Jun 12, 2006 9:03 pm

Ray, what a diabolical question ! but relevant. It was a struggle at first, the valve fought, but hopefully, yes it was cleared of its internals. I'm not saying different.

When a doubt arose Friday, we stripped both feeds from the m/c down to the first T (bout a meter) where they go into one , reamed them with welding wire, blew compressed air thru & noticed better flow from back to front than front to back on one of those. Replaced that length of copper tube. No diff to pedal before or after.

Tonight, separated system front & rear as suggested. After bleeding, rear locked OK. Isolated m/c had already locked OK. Front o/s continuing to pump lots of air-suspect faulty wheel cylinder or bleed valve ? Pursuing possibilities for both.

Post by **ronwilson** » Wed Jun 14, 2006 11:36 pm

Halleluia !!

Tonight with 2 recon Marina front brake calipers (mounted with bleed nipples upper most) got pedal on the isolated front brakes ! Linked up the whole system with servo and new m/c cover and got pedal! Still unsure what actually did the trick-so learning not wholly traceable but absolutely over the moon. Now should get it thru MOT next week and make the Nationals. Whew!

FWIW: Bought LH & RH recon Marina front brake calipers and fit the o/s (looked most dodgy) late yesterday. Bled the system but still no pedal. Tonight, remounted the caliper so the bleed nipple was uppermost on both new fronts, bled it and voila had pedal. Not sure if both calipers were faulty or if mounting the replacements w/ bleed nipples uppermost made the difference. (Looked logical to rid air that way, but had run for a year fine with bleed nipples pointing down !)

Many thanks to all who commented, and particularly Ray and John. Hope we will all find each other at the Nats.

Post by **Willie** » Thu Jun 15, 2006 7:05 pm

Glad its sorted but yes, if the bleed nipples are downwards then,in theory,you
cannot remove all the air.

Post by **rayofleamington** » Thu Jun 15, 2006 7:11 pm

Wayhey!!

Really glad you got it sorted - it sounded like a reall battle of wills.

w/ bleed nipples uppermost made the difference.

ah - well that could have made a big difference. If the air gets trapped where it can't get to the bleed nipple it is really tough to get rid of - with some driving around it will get mixed into the brake fluid and when repeating over months you can get a better bleed, but if the bleed nipples are at the highest point then you're in with more chance to get it out first time.

Post by **ronwilson** » Thu Jun 15, 2006 11:14 pm

Thanks Ray & Willie. Ouch ! Another useful learning point !

Now, here's a related but slightly different question that came up when sorting the "no brake pedal problem". Should really call this excessive brake pad wear. My apologies if this was covered elsewhere.

The rear disc pads had considerable more wear than the front pads and had worn thru about a third of their depth in only 1700 miles ! This looked excessive ! Have heard of a device called (I think) "a balancing valve" and wondered if it might be useful fitting one bx front and rear to reduce pressure at the rear calipers. Would anyone with experience of these offer some thoughts ? How they work ? Is it a useful device to sort this kind of problem and any leads for sources ? Thanks, Ron

Post by **rayofleamington** » Thu Jun 15, 2006 11:26 pm

Excessive pad wear is sometimes due to residual pressure - useful for drum brakes as they have return springs, but it is not good for disks! The Minor master cylinder has a cup washer/seal near the outlet which (if it is working correctly) needs to be 'damaged' for using disks. Some people remove it - others make small holes in the seal.
With disks all round I can't see it will be needed at all. Therefore if your current m/c has some kind of return valve you may want to remove it.

Is it a useful device to sort this kind of problem and any leads for sources ?

Overbraking on the rear is a common problem with any light car that is upgraded with rear disks. The normal complaint is unstable braking on bends (the rear breaks away very easy) - although braking on bends is best avoided you can't always do that!

Brake Balance valve:
Minors with rear disks are pretty uncommon so maybe the best place to find info would be a kit car forum / magazine. I know the Lotus 7 replicas often use these devices if they go for rear disks. alternatively try google.

Post by **minor_hickup** » Thu Jun 15, 2006 11:30 pm

Ronwilson, glad you go this sorted, from your PM you sounded really peaved as I would be. Nice one!

Post by **Willie** » Fri Jun 16, 2006 10:22 pm

It should be easy to prove if there is too much rear braking effort on your
system. If you do an emergency type stop do the rears lock up more easily
than the fronts? If yes then you have an undesireable set-up which could lead
to some hairy moments since,if the rears lock up first, the car is liable to go
in any direction it chooses. Fitting a suitable pressure regulator would be a
'trial and error' excercise although I seem to recall reading of such units which are actually adjustable?? Someone will advise. Assuming that the front and rear pads are of the same type why would the rears wear more rapidly than the fronts if the problem is due to residual pressure?