Spongy Brake Peddal

[the_boozer]

My oldmans commodore is doing my head in. Im trying to work out what we have done wrong? It was an est 6 now efi 6
It has disks all round was originally drum rear . What size master cylinder should it have 1 inch or 15/16? Are the boosters all the same (the booster on it came off a vn I think).
When we bleed the brakes the pedal comes up nice and hard but as soon as the engine starts the pedal goes spongy as if there is air in the system I cant find any leaks have put rubbers in all calipers and they seam to be working and not binding. Is it possible we have mismatched parts? cant say how well it stops as its not right and cant be driven .But all wheels lock and release and the brakes feel touchy when moving around the driveway
Cheers

[Sabbath']

Mine was the same, turned out to be the booster. I went from a 15/16 to 1" and it was still spongy, so i changed over the booster and it was much better. Booster was from a VR, fit onto the VH fine, except it sits further forward.

[Not_An_Abba_Fan]

I'd say its the reaction disc between the master cylinder and the booster. Where little rod comes out of the booster to push the master cylinder in, there is a small rubber disc behind that that fits in the booster. If that is missing it will do exactly what you are describing.

[Mr202]

Have to change the booster, its all to do with the bias setup.

[vkberlina]

Just another thought, did you hone the cylinders when you did the rubbers? you might be getting a slight bit of leakage past them if they arent sealing proplerly over time

scott

[the_boozer]

Thanks blokes for your help I put another booster on it the pedals good. I'd all ready done the same as Sabbath before I asked and think Not_An_Abba_Fan explained what Id done.
What size master cylinder should a VK EFI Calais have ?

[Tree cutter]

Should have 15/16 master cyl. Only v8 and turbo vl had inch master cylinder. Make sure the master cyl is suited to 4 wheel disc. The master cyl for drum brake rears has a valve in the fitting where you bolt the rear brake pipe into.

[SL/ENUT]

Not all V8s gained the 1 inch master cylinder.

[ari666]

my VK had 11/16th from factory.

can i heist this thread for a bit? im looking at getting a mastercylinder for my impala. in the instructions for the rear calipers it mentions i should have a 1 1/8" master cylinder, im pretty sure my booster is to suit a 1" master, will it make much difference if i go 1 1/8" to the pedal feel? does the whole 11/16th, 1" 1 1/8" refer to the internal bore size, i.e. the actual tube-thing that slides into the booster is the same size on all of them?

thanks/sorry boozer

[SL/ENUT]

the whole 1 inch/ 15/16th etc is the piston size inside the master cylinder i'm lead to believe.

[Tree cutter]

Bigger bore in your master cyl will give a firmer pedal.