Brake bleeding issues

[Heronscourt]

Before filling the brake reservoirs I decided to fit Robey's remote rear bleeding kit to make the bleeding process easier.
I did remember to remove the ball bearings when removing the existing bleed nipples.
The master cylinder, servo, calipers and brake lines are all new.
Starting the bleeding process in accordance with the manual the pedal would only go half way to the floor when bleeding the back brakes and I could only get a small quantity of fluid out.
When bleeding the front brakes the pedal could be depressed right to the floor and a full flow of fluid could be observed.
I'm unhappy with the inability to achieve the same pedal movement and fluid discharge at the rear but can't work out why.
The workshop manual shows the hydraulic pipework connections to the servo cylinder being reversed for later 4.2 models. I've piped up as the connections were prior to strip down. Barratt's catalogue shows the same servo part number for all 4.2 models so does the change in connections make any difference and if not why did jaguar make the change.
I'm currently struggling to adequately bleed the system to my satisfaction and any suggestions would be welcome.
Incidentally one of the tapers on a Robey bleed nipple was allowing a tiny drip so a swapped it out for a nipple with a ball bearing which cured the drip. I've heard of similar problems with the Robey kit.

[Allrand]

If you've already bled the front brakes you should get only half pedal travel at the rear because the front brakes are working. That's the fail safe design of dual circuit mc's, to allow one circuit to carry on working when the other fails.

[Dr-G]

I never understood how an open bleed nipple was any different from a major fluid leak, which should cause the offending circuit to be isolated, thereby making effective bleeding very dfficult.

I always use a pressure bleeding system (which effectively bypasses the fail-safes), and have never had an issue.

[mgcjag]

4.2 cars have duel line brakes with safety features.....the safety feature is the reaction valve setup and vacume operated servo...but it relies on engine running or stored vacume in the resevac tank.....the failure device comes down to the servo piston operating the scuttle in the slave cylinder....without vacume his dosnt work.......so under normal brake bleeding you have no vacume...open a bleed nipple and you should be able to push fluid through....Steve

[Allrand]

I never understood how an open bleed nipple was any different from a major fluid leak, which should cause the offending circuit to be isolated, thereby making effective bleeding very dfficult.

I always use a pressure bleeding system (which effectively bypasses the fail-safes), and have never had an issue.

[Allrand]

There's no difference, the offending circuit is not islolated, it just doesn't work because it can't build up pressure.

[mgcjag]

Original posters problem....."can only get a small amount of fluid from rear nipple"....you need to disconnect rear out of servo or at irs flexi....then see if you get full flow.sounds like a blockage or hole out of servo to rear not drilled fully through..or servo scuttle .....don't assume your new parts are good.....Steve

[Dr-G]

There's no difference, the offending circuit is not islolated, it just doesn't work because it can't build up pressure.

From the Jaguar manual:

"A failure in the fluid line coupling the slave cylinder to the front brakes will result in the slave cylinder secondary piston travelling to its fullest extent, down the bore. this has the effect of ISOLATING THE FRONT BRAKE LINE FROM THE REST OF THE SYSTEM and permitting normal fluid pressure to build up in the rear brake line."

There's a similar paragraph regarding the rear brakes (and the swapped circuits for later cars) which I can't be bothered to type out.

So if there's a leak (or the bleed valve is open) you'll get a much reduced amount of fluid out of the bleed nipple before the other circuit applies one pair of brakes and hydraulically locks the pedal. This is exactly the problem the O/P described. As I said, the problem is completely overcome by pressure bleeding.

[mgcjag]

Not quite correct...when bleeding you wont have the car running and will have exhausted the vacume tank...so the safety feature dosnt operate and one set of front/rear wont lock up.........your copied info above from the service manual refers to how the brakes work under normal conditions with vacume and safety operating...Steve

[Dr-G]

In terms of the O/P's issue, the fail-safe description is correct whether there's vacuum or not (depending on which ports their front and rear brakes are connected to). I was quoting to illustrate the use of the word "isolated" in the manual, in response to a previous comment.

My interpretation is:

With no vacuum present, and a leak (or open bleed nipple) on the circuit connected to the front most servo outlet, under pedal pressure the servo's intermediate piston should move all the way forward under fluid pressure, expelling fluid until it comes to the limit of its travel, at which point it will stop and no further fluid will be expelled (I expect this is the circuit that the O/P's rear brakes are connected to). This would explain why their pedal only moves "half way to the floor" while bleeding the rear brakes.

In contrast, the circuit connected to the rearmost servo output line will allow fluid out of that circuit's bleeders all the time the pedal is being moved downwards (I expect this is the circuit that the O/P's front brakes are connected to). This would explain why the pedal can be "depressed right to the floor and a full flow of fluid could be observed" for the front brakes.

The O/P could try bleeding with vacuum present (engine on), but I assume this would result in both front and rear brakes having limited pedal travel.

End of the day, that's how I interpret the issue, but if the O/P has the slightest doubt about anything to do with brakes, they should get it checked out by a professional.

[Allrand]

A matter of terminology, I would interpret what the maual says as 'apparently' isolated, if it was actually isolated it wouldn't allow any fluid to be pumped out of a leaking line or the bleed nipple.

[Dr-G]

I guess in the context of normal operation, it isolates the leaking circuit from line pressure. It's not immediate or permanent, but I suppose it gets the idea across?

[cunpr]

Dear All,
I'm about to replace my master brake cylinder (with reaction valve) on my 1973 S3. I've read the manual and was happy until I read the thread above which got me a bit confused. Fitting an empty (i.e. new with no fluid in it) master cylinder and then re-filling the system shouldn't be any different to bleeding the system?
Am I right?
So I should follow the factory manual's procedure of rears first then fronts?
Many thanks in advance.

[Tommd]

Paul, there seem to be many ways to bleed the brakes. Different people use different methods with success. I have bleed my E types brakes both from full and dry, and have not had too many issues. That being said:
1. I do not pressure bleed. But I believe if you do, it would not matter whether you start at the rear or the front.
2. I do it by pumping the brakes and I have no idea where I started last when I had a dry system. It just worked. But since the MC feeds the servo which feed the front brakes on the S3, I believe it would make sense to start with the front so you can get fluid in the MC etc. If you try to start with the rear, you will still have mostly air in the MC and it will be more difficult to push the fluid out to the rear.
Tom

[cunpr]

Dear All,
For what its worth I'm done with a nice firm pedal. Here's what I did and a few observations that I've not seen mentioned before.

As i had no assistant I taped a broom handle to the brake pedal so I could "pump" from outside the car and keep an eye on the reservoirs at the same time.

my car is fitted with the extended bleed pipes on the rears so i have a an extra single T piece on the other side of the IRS under the car which has a single bleed nipple for both rears, hard pipes run from each rear bleed nipple opening to either side of this additional T piece. Good to have known that beforehand especially when i took the rear caliper covers of to find no bleed nipples. Would have saved the 30 mins or so trying to work out what was going on and following the extended pipework, not easy on the IRS.

I pre bled the master cylinder. Good tip from here.

I then pumped until I got fluid at the front LH calliper. Good tip from here for an empty MC, thanks to Tommd.

I then went about the usual procedure using an eezibleed kit - what a load of shite, the bottle didn't seal because the moulding was so poor, had to smooth down the top edge then the joint on the reservoir cap leaked. Anyway finally got it working using 8lbs or pressure. I think the time getting it working and clearing up the mess it leaves would have equalled any pumping time I saved.

I had a collector bottle hanging from each nipple as it was bled with enough new fluid in it to cover the end of the pipe, so as to act as a bubble trap

No seen mentioned before that to do the rear brakes you have to use it on the slave cylinder at least I did, pumping with the device on the master reservoir didn't result in any fluid moving to the rears. Presumably due to the dual circuitry?

Because the eezibleed fills the reservoirs under pressure they fill to the top, so when you go to replace the cap you have a reservoir filled to the brim and need to suck out 20 or so ccs before the cap and sender unit can go back in. In short brake fluid everywhere. Useful to have couple of large syringes to hand - easy for a doctor

I did the rears first then the fronts.

I was surprised at how little fluid was needed to fill a half empty system (new MC, callipers and discs at the front).

Hope this helps and adds to the knowledge base.

[mgman]

Hi, I have read the exchanges on this thread. And there are some ideas I haven't tried.
My present issue is air which seems reluctant to leave to rear callipers.
The brake travel can be considerably reduced by clamping the rear flexible (with correct tool). I have pumped the brakes conventionally and used a pistol like vacuum tool, but each time fluid runs bubble-free.
I have previously use an "Easybleed " but the usually create a mess so I just use the bottle to replenish the reservoir. I am about to have another go at it now.
As an incidental sometimes the brake pedal does not fully return. Having said all that the car does stop okay.