I am writing this post after going through sinking brake pedal issues on my BJ73 cruiser and finally finding a solution which I had not come across specifically online yet. So at least it is documented if others experience the same issue.
My BJ had poor brakes since I bought it and the front brakes would lock up easy so I was suspicious that I had no rear brakes. Whilst doing a front locker install I noticed one of the front brake pads wearing unevenly so I bit the bullet and got the following done and installed them myself.
Reconditioned front Callipers
New front rotors
New front brake pads
SS Braided lines all round (6 in total)
During the brake bleeding process I could not get fluid to the rear brakes and after investigation discovered that the LSPV was seized in the shut position. After checking prices I went with the ebay option Chinese copy ($80) over a genuine ($440), which I later regretted as you will discover.
After installing the cheap LSPV I had fluid at the rear brakes so proceeded to bleed the whole system. During this process we noticed we had the issue of a slowly sinking pedal under gentle pressure, with a hard jab or decent pressure it would hold. And after reading multiple threads on here we did the usual adjustment of rear brakes and handbrake and front wheel bearings.
We decided we may have damaged a seal in the master cylinder by pushing it too far during bleeding (by the seal going past it usual travel point in the bore and hitting some corroded bore). So a new master cylinder from a local brake shop was purchased and installed by myself.
As soon as that was done and the system bled the exact same problem was apparent. When giving the brake pedal a decent push, it would hold but when resting on the pedal it would slowly sink.
So I give the brake shop a call because I am stumped from here and sick of bleeding and they decide the new master they gave me must be faulty and proceed to get a new one under warranty and install that themselves. Exact same problem still exists. Also note that there are no external leaks at all just the sinking pedal under light pressure
At this point I start thinking that all of my problems started when I installed the Chinese LSPV so I start doing some research into how these things work and I think technically it is a LSPV-BV with the LSPV part of the unit doing the rear brake pressure adjustment but the BV (bypass valve) bit had me interested. My understanding is that the BV senses the front brake pressure (makes sense as there is a front brake line going to the unit) and if there is front brake failure (no pressure in the front brake line) it ignores the LSPV and sends full brake pressure to the rear to maximise braking capacity of the car in this emergency situation.
So my theory is if somehow this Chinese LSPV-BV is dodgy then it is creating some sort of loop where brake pressure is just going around in circles due to the BV part being dodgy (linking the front brake circuit to the rear brake circuit somehow and just circulating fluid). The only reason I thought this was because when giving the pedal a jab the BV senses good pressure and operated normally, but when resting on the pedal it was not reading good pressure (assuming front brake failure) and bypassing the LSPV and creating the loop somehow. So I assume the tolerance or operating range of the BV was incorrect somehow. All of this is my theory only so don’t take it as correct either.
So I went ahead and purchased a genuine LSPV – BV and once that was installed, presto, brakes were excellent and no pedal sinking issues at all.
In conclusion without any external leaks I had a sinking pedal problem which was the LSPV –BV and not the master cylinder or anything else for that matter.
Sorry for the long winded post but hopefully someone finds this useful someday when searching for this problem just as it was.
Any comments on my theory with the BV??? I am interested to know in anyone agrees/disagrees.
My BJ had poor brakes since I bought it and the front brakes would lock up easy so I was suspicious that I had no rear brakes. Whilst doing a front locker install I noticed one of the front brake pads wearing unevenly so I bit the bullet and got the following done and installed them myself.
Reconditioned front Callipers
New front rotors
New front brake pads
SS Braided lines all round (6 in total)
During the brake bleeding process I could not get fluid to the rear brakes and after investigation discovered that the LSPV was seized in the shut position. After checking prices I went with the ebay option Chinese copy ($80) over a genuine ($440), which I later regretted as you will discover.
After installing the cheap LSPV I had fluid at the rear brakes so proceeded to bleed the whole system. During this process we noticed we had the issue of a slowly sinking pedal under gentle pressure, with a hard jab or decent pressure it would hold. And after reading multiple threads on here we did the usual adjustment of rear brakes and handbrake and front wheel bearings.
We decided we may have damaged a seal in the master cylinder by pushing it too far during bleeding (by the seal going past it usual travel point in the bore and hitting some corroded bore). So a new master cylinder from a local brake shop was purchased and installed by myself.
As soon as that was done and the system bled the exact same problem was apparent. When giving the brake pedal a decent push, it would hold but when resting on the pedal it would slowly sink.
So I give the brake shop a call because I am stumped from here and sick of bleeding and they decide the new master they gave me must be faulty and proceed to get a new one under warranty and install that themselves. Exact same problem still exists. Also note that there are no external leaks at all just the sinking pedal under light pressure
At this point I start thinking that all of my problems started when I installed the Chinese LSPV so I start doing some research into how these things work and I think technically it is a LSPV-BV with the LSPV part of the unit doing the rear brake pressure adjustment but the BV (bypass valve) bit had me interested. My understanding is that the BV senses the front brake pressure (makes sense as there is a front brake line going to the unit) and if there is front brake failure (no pressure in the front brake line) it ignores the LSPV and sends full brake pressure to the rear to maximise braking capacity of the car in this emergency situation.
So my theory is if somehow this Chinese LSPV-BV is dodgy then it is creating some sort of loop where brake pressure is just going around in circles due to the BV part being dodgy (linking the front brake circuit to the rear brake circuit somehow and just circulating fluid). The only reason I thought this was because when giving the pedal a jab the BV senses good pressure and operated normally, but when resting on the pedal it was not reading good pressure (assuming front brake failure) and bypassing the LSPV and creating the loop somehow. So I assume the tolerance or operating range of the BV was incorrect somehow. All of this is my theory only so don’t take it as correct either.
So I went ahead and purchased a genuine LSPV – BV and once that was installed, presto, brakes were excellent and no pedal sinking issues at all.
In conclusion without any external leaks I had a sinking pedal problem which was the LSPV –BV and not the master cylinder or anything else for that matter.
Sorry for the long winded post but hopefully someone finds this useful someday when searching for this problem just as it was.
Any comments on my theory with the BV??? I am interested to know in anyone agrees/disagrees.