Spongy Brake Pedal after VT Brake Upgrade

[DJ Ice]

Just finished fitting one of these kits, COMMODORE VR VS TO VT BIG BRAKE UPGRADE SLOTTED DISCS - eBay Standard Parts, Holden, Car Parts, Cars, Bikes, Boats. (end time 30-Nov-07 11:06:06 AEDST). VT Calipers discs and braided brake lines and now have a really spongy brake pedal. Have bled all 4 brakes as per Gregorys and PBR guide and still have a really spongy pedal....like bleeding the brakes made no change.

What am I missing ????

[Not_An_Abba_Fan]

You will probably find that you will need to upgrade to the VT master cylinder and fit the spacer to the pushrod. VT brakes require more fluid volume and the VS m/cyl isn't quite up to the task.

[DJ Ice]

You will probably find that you will need to upgrade to the VT master cylinder and fit the spacer to the pushrod. VT brakes require more fluid volume and the VS m/cyl isn't quite up to the task.

Thanks for your input but I think something else might be off (But you could be so right). I've gone from a really firm high pedal to a very spongy low pedal which improves with a few pumps.

[hako]

You may need to drive the car for a few miles to wear the brakes in before the pedal becomes firm. If you have ABS you may need to bleed the ABS unit with a scanning tool.

[Not_An_Abba_Fan]

The low pedal would be the amount of fluid required to push the twin pistons of the calipers out, pumping it just pushes the volume through the lines until you have enough there to push the pistons out. Unless you have removed the ABS module or let the reservior run dry, it shouldn't be necessary to bleed the ABS module.

When you bleed the brakes, the best way to do it is to have two people, one to push the pedal, one to open the bleed nipple on the caliper.

Attach a clear hose to the bleed nipple with the other end in a bottle.
Open the nipple and have your assistant push the pedal down half way.
Close the nipple and release the pedal.
Repeat until you have a steady stream of fluid through the clear hose, with no bubbles.
It is important not to push the pedal all the way to the floor, as this will make the internal cups of the M/cyl push past their normal operating area and it may damage them. This may cause the piston in the M/cyl to bypass and the pedal will slowly go to the floor when applied.
Make sure the reservior is kept topped up.

If this doesn't improve the brake feel, then I would fit a VT M/cyl.

[hako]

Adding to the good advice from NAAF, bleeding may cause the brake fail light to be activated when you start the engine. This warning light should extinguish when you get a firm pedal and can apply firm pressure to the pedal - this should reset the light.

[DJ Ice]

Thanks guys I hope it doesn't come to having to change to VT booster and master cylinder...it will blow out the cost and I don't feel like getting custom brackets made etc.

Here is a thought....I've looked on PBR's website and see there is no 1" Master cylinder for VR with ABS, could you use the Non-ABS 1" Master cylinder and blank off the extra outlet ?? like on the VT.

[Boonz]

pretty sure you cna do that

[DJ Ice]

pretty sure you cna do that

cna ? Not sure what or who you mean.

Doh...just worked out what you meant....typo can't do that

[muttley383]

I would have thought that once the piston and pad assembly is initially bled and with the pad in contact with the rotor then bugger all fluid capacity would be required as there is no gap to fill, just pressure needed for pad to rotor contact.

I have the AP four spot set up on front and rear of a VR and I did not change the master cylinder and the pedal height feels ok to me.

[Not_An_Abba_Fan]

It will depend on the diameter of the pistons. I think the VT uses 42mm pistons. When the brakes are applied the piston moves out and the seal inside the caliper twists a bit, when you release the brake the rubber seal returns back to it's normal position and pulls the piston back with it. The amount that the piston moves isn't a problem with the standard brakes, but upgrade to the VT calipers and the amount of movement on both pistons impacts on pedal travel and feel. On a VS, the single piston is 54mm, the VT total is 84mm per side. It takes a bit more fluid to push the two pistons out than a single VS piston. Some of the aftermarket twin piston calipers only have a piston diameter of I think, 32mm or there abouts for this very reason.

DJ Ice, yes you can.

[VSIIBerlinaL67]

i got the vt brake upgrade. mine works perfect firm brake pedal. your brake prob need to be bleed again.

[DJ Ice]

Update, Sent the car into the mechanics...they all scratched their heads and couldn't believe it. Anyway they flushed the brake fluid and bled the system again, result firmer pedal but still a lot of travel.

Spoke to the guys who supplied the upgrade kit...they swear that it will improve heaps after bedding in the pads...time will tell...I'll post the result after a few hundred km's.

Also after searching the forums common thing I've found....VS's seem to handle the upgrade better than the VR's for some reason, different master cylinder...hmmm

[markovr]

This is what you need http://forums.justcommodores.com.au/...-question.html

[Surfwagon]

Glad to see you worked out the spongy pedal.
Un fortunately the VT upgrade does have a longer pedal travel than standard VB-VS calipers due to the 2 large 42mm pistons in the VT caliper, the HSV VT caliper uses 40mm pistons and the VN-VS GTS/GrpA caliper uses 38mm pistons.
So you can see that more fluid is required the move the pistons in the VT caliper.
The easiest option on a VS to regain pedal travel is to fit a spacer and VT master cylinder.
I know RaceBrakes in Sydney sell the spacer, about $55.00.
The spacer is required because the pushrod depth in the VT Master is about 10mm shallower than that of the VS master.
Another option is to adapt a whole VT mastercylinder/booster combo.
A few have done this them selves and again RaceBrakes also sell this ready to fit , About $590.00.

SW

[Cheap6]

Interesting with the piston sizes.

For the piston areas:

GTS/GRPA: 38mm X 38mm X 2 = 2888mm^2

Standard VB-VS: 54mm X 54mm X 1 = 2916mm^2

VT: 42mmX 42mm X 2 = 3528mm^2

Which explains why GTS/GRPA can use the standard M/C and VT on VB-VS causes a problem.

[Surfwagon]

Nice calculations, it really shows the difference between the different caliper/piston combos.

[Cheap6]

Nice calculations, it really shows the difference between the different caliper/piston combos.

Thanks . Obviously I haven't included any pi/4 term, so they are not true piston areas but it does illustrate the comparative differences/similarities.

The true picture is worse because the volume change is piston area X displacement so the movement of the pistons and the caliper on the slides needs to be included ie. the volume change is: (pi/4) X (dia. ^2) X (number of pistons) X (2) X (the clearance between each pad and the disc surface) - for a sliding caliper only.

[Spaced]

I'm running VT brakes on a VS. Until my brakes were bedded in they were crap, now they are fantastic. Pull up really really well and when I slam them on (only had to once) it near pulled up on the spot.

[Smitty]

Just finished fitting one of these kits, COMMODORE VR VS TO VT BIG BRAKE UPGRADE SLOTTED DISCS - eBay Standard Parts, Holden, Car Parts, Cars, Bikes, Boats. (end time 30-Nov-07 11:06:06 AEDST). VT Calipers discs and braided brake lines and now have a really spongy brake pedal. Have bled all 4 brakes as per Gregorys and PBR guide and still have a really spongy pedal....like bleeding the brakes made no change.

What am I missing ????

i have been thru all the posts here
and as I see it (correct me if wrong)
if you have this problem of a spongy or 'long' pedal you.....
bleed again and bed in pads
or
get a VT master/booster combo (with adaptors for the lines) and modify it to suit
or
get a VT master/booster setup from a brake place (like Racebrakes ) already modded and bolt it in

whats curious to me is that some with VT front rotor/caliper setups don't have a problem and
some of you do have the problem (with the same setup)..that I don't get !
whats different?



cheers

[Surfwagon]

Maybe It could have to do with the age of the rear brake hoses.
Most of the VT upgrades come with 2 front braided hoses but leave the rears standard.
After fitting my Ford AU calipers (40mm pistons) on my 330x28mm discs my pedal travel was a bit longer and not that firm.
Did all the usual, bleeding, bedding in, discs skimmed flat and new pads in the rear but nothing changed until I replaced all 3 rear hoses with braided lines then all was fine.
Pedal travel was still a little more than stock but nothing to worry about but pedal is firm.
Nothing has changed since adding the HSV VX Clubby brakes, 330x32F & 315x18R, the clubby calipers have the same piston size as the Ford calipers but they allow a wider disc and the brakes are awesome.

[DJ Ice]

whats curious to me is that some with VT front rotor/caliper setups don't have a problem and
some of you do have the problem (with the same setup)..that I don't get !
whats different?

cheers

I agree...strange but pedal feel is subjective what one person finds to be OK another will say is terrible.

Update 2 to the story...the brakes are starting to bed in and feel much better but still way to much travel.
Spoke to guys at Race Brakes in Melbourne who said they could do a modified VS Booster and VT Master cylinder combo for $360 plus freight which seems very reasonable. They canned the whole VT brake setup saying the calipers flex too much and the resulting poor pedal makes it a bad choice.

[markovr]

They told me that too but mine are fine .....my car is a vr with slotted vt front brakes and qfm pads all round .....a modified bracket on a vt brake booster to fit a vn/p/r/s and vt master cylinder and vs resevoir and front bracket to suit ..been in for 3 years and never touched after initial instal...Ive done plenty of them ......Ive got one spare complete setup which is a bolt up to your car to sell 200+ 20 post

[DJ Ice]

They told me that too but mine are fine .....my car is a vr with slotted vt front brakes and qfm pads all round .....a modified bracket on a vt brake booster to fit a vn/p/r/s and vt master cylinder and vs resevoir and front bracket to suit ..been in for 3 years and never touched after initial instal...Ive done plenty of them ......Ive got one spare complete setup which is a bolt up to your car to sell 200+ 20 post

Thank you for the offer might take you up on that Will PM you if I decide to go that way.

[DJ Ice]

Final Update: Fixed...My stupid fault I didn't install the outer brake pads correctly the u shaped part of the clip was crushed in transit and I didn't pickup on that and as a result I didn't install them correctly. Once I worked out the correct way to fit them I reinstalled them and now I have less travel and firmer brakes...much better braking power than the old VR setup. BTW Nice pads supplied with the kit, low dust and no fade. They are a generic pad similiar to Bendix Ultimate.