Brakes wont bleed - GT6

[markcro]

Over the last month I have fully rebuilt the front calipers and refurbed the rear drums. After fitting them back to the car about 2 weeks ago I accidentally let the brake reservoir run dry and it sucked in alot of air.
So last night was time to bleed the brakes properly and put the wheels back on the car for a spin. But after a full litre of fresh brake fluid and over an hour of bleeding I still cannot get a firm brake pedal!!  :(
If I push the pedal it will firm up and stop about halfway, but if I then release it and wait 10 seconds, and then press the pedal, it will then go all the way to the carpet again.
Any ideas?

[Velocita Rosso]

Servo or not?

[Nilfisken]

Saying you used a litre of brake fluid did you still had bubbles in the line for so long? One thing that happened to me while redoing the front last year was that when I bled the brakes I pushed the pedal all the way to the floor during bleeding and since that is not a position the master cylinder piston seen in years some crud in the piston bore ended up damaging the seals in the master. I changed the master cylinder and all was good again. Just like you I had to pump the brakes to get a firm pedal and soon after it was back to the floor again, found out while test-driving...   :o

[John Bonnett]

Silly question Mark but is the handbrake on?

I recently did this job on my car and it is definitely best done with two people. You need one to operate the pedal and the other to spot for bubbles. I probably used a litre of fluid before the pedal was firm. I started with the offside rear, the longest run and then in order of pipe length from the master cylinder. Although the pedal is good with the engine off it is quite spongy once there is vacuum in the servo. But I am using silicone brake fluid!

[markcro]

Cheers guys.  Hopefully not the master cylinder. But it wouldn't surprise me as the more that I do the more that I find that needs doing!  :B

- She has a servo.
- Handbrake is off - should I put it on?
- There was air in the nearside front and rear brakes, but after that none came out. But the pedal still wont firm up. I will have another go on Sat as I did not run as much fluid through the near side, so I will go over it all again.


(And while I was doing the above the fuel pump started leaking!  :-/ ;D  You just have to laugh!)

[Paudman]

Bled mine after a master cylinder rebuild as a solo operation - no bubbles seen, but the first test drive revealed a lot of sponginess at the pedal. I enlisted the help of 'er indoors and whilst I saw no visible bubbles on the second bleeding, the pedal has really firmed up. It's the only way to do it (and believe me I've tried all manner of air powered accoutrements supposedly designed to help.)

[Freebird]

There's your answer then Markcro, you need the help of Paudman's missus - simples.

[markcro]

Can I borrow her Paudman?  ;)

[Velocita Rosso]

We bleed the brake with the engine running to establish full working with servo
If its doing  what you say its doing then it could be the master cylinder sucking in air as you pump eg seals have gone or wear on bore
No matter what you do if it looks as though you have succeded and then its still spongy after a while ..then its as said ..sucking in air through M/C
Belt and braces ...we (2) pump and watch for bubbles in a glass WITH a non return valve bleed hose AND pump ,hold down ,undo and tighten screw,let up, pump ,hold down, undo and tighten screw .......and on and on and on
If after all that and it still is spongy.............new cylinder  

[markcro]

I bled front left, front right, rear left, rear right as per the book. How much fluid should be enough to purge each corner? i.e. would one full reservoir per corner be enough?

[GT6 M]

I just leave the nipples open, and let gravity doo the rest.
always has worked for me.

M

[Deleted User]

air rises? Sounds to me you have air ingress getting in somewhere?

[Tim Bancroft]

Got to agree, go through all joints and check that tight.

[Paudman]

markcro wrote:

Can I borrow her Paudman?  ;)

She may firm up more than the pedal, if that's alright....

[junkuser]

Sounds as if it could be the master cylinder cup is worn.
Try stamping rapidly on the pedal and see it pressurises correctly.

[markcro]

490 wrote:

She may firm up more than the pedal, if that's alright....

Ha ha ha!  8)

[markcro]

junkuser wrote:

Sounds as if it could be the master cylinder cup is worn.
Try stamping rapidly on the pedal and see it pressurises correctly.

Yes when I pump the pedal rapidly it does build pressure and the pedal can't be push below halfway as it's solid. But release it and then press again and it will go all the way to the floor.

[TimW]

sounds like you need either new seals in the M/C to me. or a new M/C

[Velocita Rosso]

Also another tip if the M/c has gone, take top off cylinder an watch for bubbles

[junkuser]

"Yes when I pump the pedal rapidly it does build pressure and the pedal can't be push below halfway as it's solid. But release it and then press again and it will go all the way to the floor."

Stamping on it is not curative, just diagnostic.
Rapid movement of the piston causes the edge of the cup(s) out against the wall of the cylinder and it will stay there as long as pressure is maintained.
With normal brake application this may not happen so the piston will proceed down the cylinder without resistance.
Replacing the master cylinder is probably the best option if you are not experienced in overhauling them.
What price safety?

[tiggrr1]

A little trick that has worked for me on more than one occasion in the past.

Pump your brake pedal until it goes solid then jam it in place with a piece of wood/broomstick or whatever falls to hand.

Leave it for a few hours or overnight.

Take out the wood and usually you will have a firm pedal.

I don't know how or why it works but can only assume it allows trapped air back up into the master cyl  :-/