Rear Brake Bleeding 1971 Series 3 V12

[dooley1414]

Hi all just a simple question when bleeding my brakes all round I got a full travel pedal whilst bleeding the front brakes but whilst bleeding the rear brakes only got half a pedal and it comes to a stop is this normal even though I am getting air from out of the air lines.... Many Thanks In Advance.... Des. Oh just to add I have replaced the master cylinder slave cylinder and servo with new parts.

[mgcjag]

Hi..whats your name...please try to put it in with your car model in the signature are.....re the brakes..i assume you are saying you only get half a pedal with the bleed valve open.....this does not sound right....are you getting fluid out with each half press.... Steve

[dooley1414]

Hi.... Sorry my name is Des.... from Suffolk....

Yes I do get fluid from each pump from the pedal.... although a half travelled pedal.... No it doesn't seem right to me either.... but once bled.... I have a decent feeling pedal! ? But as the car is not running yet as I have the carburettors and cam covers off I can't start it up and run it down the road to see if it stops nicely.

[lowact]

Try opening the bleed screw a bit (lot) more. This worked for me.

[mgcjag]

Hi Des....are you just re topping up the existing fluid..or replaceing all of it....thinking you may have dirt in there blocking the pipes a bit.....or possibly the rear flexi pipe collapsing inside...it has happened before acting as a kind of valve....so as you pressure the system the pipe collapses and stoppes the flow.....Steve

[Dave123m]

Hi Des,

This is not uncommon when changing the Servo as I had exactly the same problem.

If you have access to a 'pressure system' such as the Gunson Easibleed it makes this very easy as it pushes the fluid through the system from the reservoir to the rear brakes.

Without it you will get exactly what you're experiencing where the pedal is prevented from going full travel due to the pressure in the front brakes as the proportioning valve won't (by its very design) allow all the fluid to the rear brakes.

With an Easibleed to get the fluid to the rear brakes & the assistance of someone to do the final bleed with the pedal this shouldn't be more than a 10 - 15 minute job.

Good luck.

Dave

[jagwit]

I recently re-commissioned an S3 for a friend. This job entailed removing and overhauling the brake calipers. Most of the brake fluid ran out of the master and servo cylinders (which were both still good). Once all the hardware was mounted, I started the bleeding process using my trusted method of having a helper operate the brake pedal (on my command) while I opened and closed the brake nipples:
1) Open nipple and I shout "down" and helper pushes pedal down;
2) I close nipple and I should "UP" and helper releases pedal (Slowish, as not to suck air past the seals in the caliper)
3) repeat 1- 2

Major struggle.... My trusted method did not seem to produce any meaningful results.

I had started on the rears but after a long time with no good results, it seemed to me as if the front brakes "wanted" to be bled first, which then went very well. Going back to the rears, no go.

Eventually, I disconnected the brake line under the body, before the rear axle and with my finger opened and closed the brake line coming from the front with my finger while increasing the cadence of "down" and "up", until eventually I had a positive squirt of brake fluid coming from the brake line and then reconnected it.

I then removed a nipple on the rear calipers completely and again, with my finger opened and closed the nipple hole, again with a faster cadence of "up" and "down". Once I had a positive squirt coming out that caliper, I re-intalled that nipple and did the same on the other caliper. Only then could I proceed with bleeding normally by opening and closing the nipples.

[dooley1414]

Hi Des....are you just re topping up the existing fluid..or replaceing all of it....thinking you may have dirt in there blocking the pipes a bit.....or possibly the rear flexi pipe collapsing inside...it has happened before acting as a kind of valve....so as you pressure the system the pipe collapses and stoppes the flow.....Steve

Hi Steve I have replaced the rear flexi pipe I think it was collapsed as when I put a pressure line on it when it was off the car no pressure was going through no air at all but still only seems to be half a pedal when bleeding. Des.

[dooley1414]

Hi Des,

This is not uncommon when changing the Servo as I had exactly the same problem.

If you have access to a 'pressure system' such as the Gunson Easibleed it makes this very easy as it pushes the fluid through the system from the reservoir to the rear brakes.

Without it you will get exactly what you're experiencing where the pedal is prevented from going full travel due to the pressure in the front brakes as the proportioning valve won't (by its very design) allow all the fluid to the rear brakes.

With an Easibleed to get the fluid to the rear brakes & the assistance of someone to do the final bleed with the pedal this shouldn't be more than a 10 - 15 minute job.

Good luck.

Dave

Hi Dave I do have some sort of easy bleed system connected to an airline which didn't result in much so went back to the helper side of things with the flexi pipe replaced at the rear the fluid did come through very very slowly but not as the fronts did they bled as they should or as you would think they would but the rear fluid did pump through very slowly but still with that half pedal feel not sure if it should be like that or not but seem to have got all of the air out but will only know if it will stop the car when I have it on the road for his first test drive.... the brake pedal does feel good though.... Des.

[dooley1414]

I recently re-commissioned an S3 for a friend. This job entailed removing and overhauling the brake calipers. Most of the brake fluid ran out of the master and servo cylinders (which were both still good). Once all the hardware was mounted, I started the bleeding process using my trusted method of having a helper operate the brake pedal (on my command) while I opened and closed the brake nipples:
1) Open nipple and I shout "down" and helper pushes pedal down;
2) I close nipple and I should "UP" and helper releases pedal (Slowish, as not to suck air past the seals in the caliper)
3) repeat 1- 2

Major struggle.... My trusted method did not seem to produce any meaningful results.

I had started on the rears but after a long time with no good results, it seemed to me as if the front brakes "wanted" to be bled first, which then went very well. Going back to the rears, no go.

Eventually, I disconnected the brake line under the body, before the rear axle and with my finger opened and closed the brake line coming from the front with my finger while increasing the cadence of "down" and "up", until eventually I had a positive squirt of brake fluid coming from the brake line and then reconnected it.

I then removed a nipple on the rear calipers completely and again, with my finger opened and closed the nipple hole, again with a faster cadence of "up" and "down". Once I had a positive squirt coming out that caliper, I re-intalled that nipple and did the same on the other caliper. Only then could I proceed with bleeding normally by opening and closing the nipples.

Hi Phillip it seems that I've got the same problem that you did but with replacing the rear flexi brake hose I did get some fluid through not as good as the front but I think the rear flexi brake pipe had collapsed or a bad blockage as to when I put an airline on the Old flexi pipe no air was travelling through it at all but I seem to have a good pedal now but when I was bleeding it still had that half pedal feel which I didn't definitely not get on the front so only time will tell if I have proper braking when it goes for its MOT.... Des.

[Dave123m]

Hi Des,

Great news you have a good brake pedal feel & that replacing the rear flexible appears to have improved the situation.

My front brakes bled exactly as I would have expected them to (as it appears did yours) however I replaced all my flexible lines with Goodrich when I re-commissioned the car in 2012 so was reasonably confident these were not the issue for me but I know they are prone to failure with age.

When I did my Servo I had the luxury of a ramp so could test the performance of the rear brakes and found there was very little difference in the pedal feel when the rears were working or not however if there is air in the rears then the pedal stop will generally be inconsistent so I'm hopeful you've sorted it now.

Good luck with the MOT, what could possibly go wrong?!?

Regards,

Dave

[mgcjag]

Hi Des...giving this a bit more thought and looking at the brake system diagrams in the service manual i dont think you will get full brake travel.....iv never noticed it before but then if bleeding i would always start at the furthest away and work to the front.....have a look in the service manual there are very good detailed diagrams of all the brake failure senarios......the one relating to this is the failure of the rear brake line...in your case opening the bleed valve.....the master cylinder pushes fluid to the slave.....the slave piston moves forward but continues to block fluid loss to the rear but pressurises the front......so you dont get total brake failure...so you would not get full pedal stroke...Dave did mention this above.......Steve

[dooley1414]

Hi Des...giving this a bit more thought and looking at the brake system diagrams in the service manual i dont think you will get full brake travel.....iv never noticed it before but then if bleeding i would always start at the furthest away and work to the front.....have a look in the service manual there are very good detailed diagrams of all the brake failure senarios......the one relating to this is the failure of the rear brake line...in your case opening the bleed valve.....the master cylinder pushes fluid to the slave.....the slave piston moves forward but continues to block fluid loss to the rear but pressurises the front......so you dont get total brake failure...so you would not get full pedal stroke...Dave did mention this above.......Steve

Hi Steve.... I have heard about this Brake Failure System and was hoping this was the case of my half traveling brake pedal? I think with your explanation.... you have made me quite faithful in my Brake System.... Thank You.

Des.