18 replies to this topic

[biga064]

Hi can you plumb your master cylinder lines wrong ?  I dont seem to be able to get good pressure to the fronts 500psi manually and 700ish boosted but pedal goes to the floor when boosted ! 

Protex 7470 master disc to disc  ,

 

Have bleed numerous times !

Calipers right way up !

Clamped lines up pedal is rock hard take clamps of front calipers pedal goes to the floor !

 

 Thanks Again !!

 

[Balfizar]

Hi can you plumb your master cylinder lines wrong ?  I dont seem to be able to get good pressure to the fronts 500psi manually and 700ish boosted but pedal goes to the floor when boosted ! 

Protex 7470 master disc to disc  ,

 

Have bleed numerous times !

Calipers right way up !

Clamped lines up pedal is rock hard take clamps of front calipers pedal goes to the floor !

 

 Thanks Again !!

Air in the calipers or capacity of calipers is too big for M/C

What calipers are you running?

[biga064]

Pbr cast aluminium Fronts  Rears Vn .

Edited by biga064, 04 April 2021 - 03:13 PM.

[ljv8]

When I built my LC and put in all new lines, master etc so started with a dry system I bled my brakes multiple times but had a poor spongy pedal.  I went through nearly two bottles of brake fluid but had hopeless brakes.  Strange thing was no air bubbles were coming out so appeared to be properly bled.

 

Searching this forum a few people talked about bench bleeding the master cylinder.  I did that and wow the amount of air trapped in the master was an absolute eye opener.  Back on the car a final bleed and brakes came good.  No more issues.  Not sure if that’s your issue pedal going to the floor with the booster on is weird but may be worth a try.

 

How to bench bleed

https://youtu.be/9bEPrO99B5A

 

Kit I used to do it

https://www.totaltoo...set-ttiautot022

 

 

 

[LXCHEV]

Interesting video on the bench bleeding.

[Dr Terry]

If the pedal goes rock hard if you clamp only the front 2 hoses, then your problem definitely lies with the front calipers.

 

Can you post a photo of them mounted in the position.

 

Dr Terry

[biga064]

Hi Dr Terry ,i cant get a photo to load , but yes its something with front  calipers , there rubbish !!!! I ve been in contact with Datto he has given me a couple of things to try but nothing is working ?? Thanks 

[LJ RB30]

Hi

I started on my LJ with single diaphragm booster, standard LJ master with WB front calipers on VN disc's & VL disc & caliper rears.

Brakes were crap.

Pedal didn't hit the floor but went bout 7/8 the way there & pulled the car up pretty ordinary, definitely no major improvement over LJ disc's.

After much rebleeding & head scratching i finally crunched the numbers on caliper & master piston sizes.

Basically the master was same size bore as a VL but the WB caliper has a piston with approx 25% larger surface area than the VB-VL caliper.

I found a set of VL calipers & drilled the front adapters to suit.

Greatly improved the braking  ( then you can go faster then you want better braking )

I put it down to basically the fronts were taking the bulk of the fluid but with not enough pressure to make them bite hard enough before the rears then did their bit. 

Hope this helps 

[biga064]

Hi

I started on my LJ with single diaphragm booster, standard LJ master with WB front calipers on VN disc's & VL disc & caliper rears.

Brakes were crap.

Pedal didn't hit the floor but went bout 7/8 the way there & pulled the car up pretty ordinary, definitely no major improvement over LJ disc's.

After much rebleeding & head scratching i finally crunched the numbers on caliper & master piston sizes.

Basically the master was same size bore as a VL but the WB caliper has a piston with approx 25% larger surface area than the VB-VL caliper.

I found a set of VL calipers & drilled the front adapters to suit.

Greatly improved the braking  ( then you can go faster then you want better braking )

I put it down to basically the fronts were taking the bulk of the fluid but with not enough pressure to make them bite hard enough before the rears then did their bit. 

Hope this helps 

Hi thanks i have considered this but im told  a1inch should be good ?? maybe i should try a 1 1/16 master for more fluid ? thanks 

[LJ RB30]

I looked at that option but decided to do calipers because I would have had to redo pipe work on the master.
Basically I tried to replicate the brake system from the one vehicle ie commodore instead of mix & match & hope for the best as I did in the beginning.
So many variables in brakes, pedal ratio, m/c caliper & pad sizes & compounds etc.
There is always a trade off somewhere between braking action & pedal feel & travel.
Small changes can make big differences.
Obviously you want the best braking with minor pedal input.

[yel327]

I just used a VT cylinder with a Harrop adapter to the booster, has a stepped bore. Running AUII calipers and HQ-WB drums. HG booster.

Edited by yel327, 08 April 2021 - 12:17 PM.

[Dr Terry]

I don't think that you need to re-invent the wheel here, it's basically a stock brake system, with the addition of 2 small rear callipers. The problem appears to be in the front callipers.

 

You say the fronts are PBR cast aluminium, do you mean the HX/HZ style with the thick sliders ? A 1-inch m/cyl & a normal booster will be fine.

 

Dr Terry

[76lxhatch]

Just to throw another possibility out there, I binned a pair of HZ aluminium calipers a while ago due to a similar issue. Came to the conclusion that they were just old and over-stressed causing too much caliper flex, replaced with cast iron HQ calipers (exact same pistons) and solved the problem. If you have access to a different pair of calipers even just for a quick test it might help with the troubleshooting.

[biga064]

I don't think that you need to re-invent the wheel here, it's basically a stock brake system, with the addition of 2 small rear callipers. The problem appears to be in the front callipers.

 

You say the fronts are PBR cast aluminium, do you mean the HX/HZ style with the thick sliders ? A 1-inch m/cyl & a normal booster will be fine.

 

Dr Terry

Hi yes known as Dattos door stop calipers !! Booster is pbr XA Ford 8inch dual diaphram booster !

Edited by biga064, 08 April 2021 - 06:06 PM.

[biga064]

Just to throw another possibility out there, I binned a pair of HZ aluminium calipers a while ago due to a similar issue. Came to the conclusion that they were just old and over-stressed causing too much caliper flex, replaced with cast iron HQ calipers (exact same pistons) and solved the problem. If you have access to a different pair of calipers even just for a quick test it might help with the troubleshooting.

I have two pairs of the exact same calipers and both pairs do the exact same thing ? What are the chances both pairs are ted ?? 

[claysummers]

Not rekevsnt, just a waffle about those crappy HX calipers. Driving on the SE Freeway in my HX van BITD. Nasty vibration and pulling front right. One of the slider bolts has fallen out and caliper is binding and locking up on the disc. Towed to Murray Bridge Holden dealer and replaced with high tensile bolt all good. Tow car was a HR Ute ffs. Car trailer start a to fish tail and there is no accelerating out of it in this case. Just coast and cross everything. End dribble.

early Holden nut

[bat 53]

those alloy calipers with the big sliders are girlock not pbr 

i use those with commodore rears with vt - vz mastercyl

[Dr Terry]

those alloy calipers with the big sliders are girlock not pbr 

The ones that we are talking about are PBR. Check out this post & it's the 2nd caliper from the beginning.

 

http://www.gmh-toran...rviki-calipers/

 

Dr Terry

[bat 53]

ok my bad