Front brake fade - need advice
- Thread starter gdouaire
- Start date
- Status
Really, really dumb stuff on my part - combined with poor documentation coming from MBM.
I took it from the begining, revising my understanding of the prop valves.
I had an old GM proportioning valve laying around. I looked at it and took it apart completely, in order to understand how the low pressure valve works. It's very basic, I have seen diagrams on the web, but seeing it in real is very interesting and makes it easier to understand.
I then proceeded to remove the MBM PVC prop valve from the Cultass and went ahead to disassemble it - removing the adjusting valve and also the switch.
So the MBM PVC valve has no moving parts inside - no "piston" that will move if you have more pressure in the front circuit then in the rear circuit. Actually, that makes a lot of sense, because shutting down the rear valve would likely trip the system every time. Furthermore... Upon inspection with a very small allen key that I used to probe the oil passages internal to the PV, it looks like the front and rear circuits are COMPLETELY independant. There's no connection between the two. I'll continue playing with it with air to understand how this is routed internally and make sure this is 100% accurate (and clean the thing at the same time).
Finally (and consequently), the switch on the PV is *not* used for the break warning light - it's a plain light switch which most of use don't need.
This was about the importance of understanding how things work before playing with them.
Now, onwards...
Bleed the system (again), making sure no bubbles trapped in the system. I am under the impression that the symptoms seen before may have been linked to air trapped behind the light switch, which is a "dead end" oil passage and will never bleed by gravity unless the switch is partially unscrewed...
I took it from the begining, revising my understanding of the prop valves.
I had an old GM proportioning valve laying around. I looked at it and took it apart completely, in order to understand how the low pressure valve works. It's very basic, I have seen diagrams on the web, but seeing it in real is very interesting and makes it easier to understand.
I then proceeded to remove the MBM PVC prop valve from the Cultass and went ahead to disassemble it - removing the adjusting valve and also the switch.
So the MBM PVC valve has no moving parts inside - no "piston" that will move if you have more pressure in the front circuit then in the rear circuit. Actually, that makes a lot of sense, because shutting down the rear valve would likely trip the system every time. Furthermore... Upon inspection with a very small allen key that I used to probe the oil passages internal to the PV, it looks like the front and rear circuits are COMPLETELY independant. There's no connection between the two. I'll continue playing with it with air to understand how this is routed internally and make sure this is 100% accurate (and clean the thing at the same time).
Finally (and consequently), the switch on the PV is *not* used for the break warning light - it's a plain light switch which most of use don't need.
This was about the importance of understanding how things work before playing with them.
Now, onwards...
Bleed the system (again), making sure no bubbles trapped in the system. I am under the impression that the symptoms seen before may have been linked to air trapped behind the light switch, which is a "dead end" oil passage and will never bleed by gravity unless the switch is partially unscrewed...
Red = rear brake lines, red arrow shows where the master cylinder input is for rear brakes.
Yellow = front brake lines, yellow arrow shows where the master cylinder input is for front brakes.
Yellow line outlined in blue = place where air bubbles will stay FOREVER unless you specifically bleed them via the 1/8 NPT fitting.
Yellow = front brake lines, yellow arrow shows where the master cylinder input is for front brakes.
Yellow line outlined in blue = place where air bubbles will stay FOREVER unless you specifically bleed them via the 1/8 NPT fitting.
Attachments
Starting from the beginning, when you say you have no stopping power if you shut off the rear brakes do you mean the pedal goes to the floor or that you have a good feeling pedal that is completely ineffective?. Does pushing the pedal feel like standing on a brick or can you feel the booster helping?.
The pedal going to the floor means you have air trapped in the system somewhere, if not then your problem lies elsewhere. If pushing the pedal is like kicking the Hoover Dam then the booster isn't doing its job. It could be it needs setting up properly for your hybrid system, or it is defective.
If the pedal feels normal but nothing happens then you could have something partially obstructing the brake lines, a piece of dirt or metal from a poorly finished brake line, or a collapsed flexible hose.
If your pedal goes to the floor or had excessive travel then bleeding the system as described above should shift trapped air. If you have a rock solid pedal check out the booster. If it is a vacuum servo make sure the vacuum pipe is hooked up properly - don't take offense, sometimes we miss the obvious.
If the pedal feels good but nothing happens check out the brake lines this way (you will need an assistant);
Undo each brake line in turn to the front brakes starting at the master cylinder, and have your helper push the pedal firmly. You should get a rush of brake fluid each time. if you come across one where you get nothing or a slow trickle you have found your obstruction. This gets messy easily, wear eye protection and gloves, keep a good supply of water to rinse off any brake fluid that spills onto paintwork immediately.
Semi ceramics do need more pressure than normal pads, but for normal driving they should be fine. They have a place in pure race and rally cars, but in those applications they need to get hot before they work effectively. They are designed to work that way with rotors designed to work with them, If you ever watch world rally racing you will see the brakes glowing orange on the cars, but they have no place on a car driven on public roads.
Roger.
The pedal going to the floor means you have air trapped in the system somewhere, if not then your problem lies elsewhere. If pushing the pedal is like kicking the Hoover Dam then the booster isn't doing its job. It could be it needs setting up properly for your hybrid system, or it is defective.
If the pedal feels normal but nothing happens then you could have something partially obstructing the brake lines, a piece of dirt or metal from a poorly finished brake line, or a collapsed flexible hose.
If your pedal goes to the floor or had excessive travel then bleeding the system as described above should shift trapped air. If you have a rock solid pedal check out the booster. If it is a vacuum servo make sure the vacuum pipe is hooked up properly - don't take offense, sometimes we miss the obvious.
If the pedal feels good but nothing happens check out the brake lines this way (you will need an assistant);
Undo each brake line in turn to the front brakes starting at the master cylinder, and have your helper push the pedal firmly. You should get a rush of brake fluid each time. if you come across one where you get nothing or a slow trickle you have found your obstruction. This gets messy easily, wear eye protection and gloves, keep a good supply of water to rinse off any brake fluid that spills onto paintwork immediately.
Semi ceramics do need more pressure than normal pads, but for normal driving they should be fine. They have a place in pure race and rally cars, but in those applications they need to get hot before they work effectively. They are designed to work that way with rotors designed to work with them, If you ever watch world rally racing you will see the brakes glowing orange on the cars, but they have no place on a car driven on public roads.
Roger.
Rusty, are you sure your not thinking of Carbon, I don't know of any Ceramic pads used in racing ...
http://ebcbrakes.com/articles/ceramic-brake-pads/
http://ebcbrakes.com/articles/ceramic-brake-pads/
You might be right, I have always found standard pads and rotors just fine for my requirements, with no special break in procedures necessary.
Perhaps I had ceramic in my head thinking of its high temperature tolerances.
Roger.
Perhaps I had ceramic in my head thinking of its high temperature tolerances.
Roger.
Ok so here we go for the root cause...
Before reassembling everything, I started looking at the master cylinder and how a 89 Caprice unit works... - well well well... looked like the front & rear portion of the MC were not hooked correctly to the proportioning valve. Read a lot on the web. Yeah, things were mixed up. Re-bent the tubes so that rear/front outputs were swapped.
Once reassembled, it made more sense: the aft portion of the master cylinder, with a large diameter, which meant more break fluid volume was hooked to the front wheels. Also, made much more sense with the adjustable prop valve... when the valve is shut, you don't want it to block the first portion of the master cylinder...
And test drive proved it worked A1....
Before reassembling everything, I started looking at the master cylinder and how a 89 Caprice unit works... - well well well... looked like the front & rear portion of the MC were not hooked correctly to the proportioning valve. Read a lot on the web. Yeah, things were mixed up. Re-bent the tubes so that rear/front outputs were swapped.
Once reassembled, it made more sense: the aft portion of the master cylinder, with a large diameter, which meant more break fluid volume was hooked to the front wheels. Also, made much more sense with the adjustable prop valve... when the valve is shut, you don't want it to block the first portion of the master cylinder...
And test drive proved it worked A1....
gdouaire said:
so when you thought you were closing off the rear brakes you were actually shutting down the front breaks...no wonder they didn't work. Glad you got it fixed.. 🙂
it's more complex than that, because most of my tests were made with the valve fully open... when depressing the pedal, the first motion sent a lot of oil to the rear brakes, which took what they needed and no more; but then, it made the remaining travelling of the master cylinder very difficult, as pressure was very high, which led to no oil pressure up front because the front wheels were hooked to the "deeper" portion of the master cylinder...
also explains the front brake "fade": after a few stops, pressure in the system build up, which in turn made it more difficult to send any oil up front...
- Status
Similar threads
