Someone please confirm or deny my theory or at least point me in a different direction.
I converted manual drum front brakes to power discs this winter.
Up until today, I've driven the car a few times and had a low, spongy pedal, which I traced down to air in the lines to the rear wheels. Bad enough that the brake light would come on if I hit the pedal to the floor.
But I must say, I could definitely feel the new power booster working correctly when I drove it. There was for sure a pedal advantage with the car running vs. when it was off.
Today, I bled the rears again and now believe that I've got all of the air out.
BUT, now with the car running, I get no power brakes. The pedal is higher and harder than before, brake light does not come on, but no noticeable difference with the booster hooked up or not. The only thing I did was bleed the rears.
The booster manufacturer recommends 20" vacuum at idle. The best I can do is about 15" with my cam. But I've read where some guys are running as low as 12" and still have power brakes?
My theory is that when I had air in the lines and spongy brakes, the booster had enough vacuum to compress the air and give me some pedal advantage. Now that I've got the air out, there is more back pressure resistance against the pedal, and I don't have the vacuum to overcome it.
Does that make any sense ????????? Or am I missing something else?
The kit was the high performance MBM with 9" single Delco style booster and 1-1/8" bore master cylinder.
1. MBM says you need 18" of vacuum, and you are less than that. 2. You're running a 9" booster, which is relatively small compared to the more typical 11" which provides more power boost. 3. Increasing bore size of master cylinder bore (your 1 1/8" vs 1") increases braking effort.
So my theory is that you have a trifecta of items, any one of which (and certainly all three together) could conspire to create a rock hard, unboosted pedal.
So I'm thinkin' you need to....get bigger diameter booster (and/or a canister), and/or add more vacuum with an electric pump, and/or or get smaller MC bore.
What does MCM tech support say about your prob? I admit they'd be a better resource than I am...and I also suspect they're gonna go with power brakes 101 as written right on their website....you need 18" of vacuum to run their booster...they 16" to 18" can be improved with a canister (again another clue that your 9" booster choice may be undersized for your amount of vacuum), and 16" or less will flat-out require an electric pump regardless of booster or canister size: http://www.mbmbrakeboosters.com/index.php?option=com_content&view=article&id=12:what-to-do
I agree with everything that crazecars says and his posts are always "well informed". Sometimes when you are bleeding the brakes the spool valve in the proportioning valve travels all the way to one end of its travel and blocks off the flow of fluid to the other brakes. Try bleeding the front brakes and this should correct itself. A vacuum bleeder can help avoid this. also make sure the rear brakes are adjusted up all the way. Hope this helps.
I'm with Neill. Sounds like the dist valve may have moved all the way to one end blocking flow. It happens if you lose pressure on either the fron or the back. I run a std sized booster with disc and only have 11" vacuum at idle. Brakes work fine. Sometimes they will tell you that if the vac builds up they will work once and then not the second time unless the vac is refreshed but I've never had this happen either so I don't know what to tell you.
are you bleeding with a helper or by yourself, I'm thinking you have an issue with the system, not the booster, I run a 9" booster with low vacuum and do not have the hard pedal issue or spongy pedal. Either you have a bad proportion valve or it traveled to far to the rear or forward or you have a wheel cylinder that isn't releasing. you should always bleed all 4 Check the rod off the pedal could be set to tight. did you bleed the MC before installing it? When I converted to disc from drum, I would go into skids when I applied too much foot pedal, I then looked into adding stock components and it was eliminated. Maybe you have too much pressure for the system and need the front brake proportion valve too, Dist. block and the hold off for the rear brakes, Are you using original Wheel Cylinders?
I checked the piston in the prop. valve, its OK. I use the pin tool to keep it centered when I bleed them. And I know from past experience the brake light will come on when it gets off center and I don't have that currently happening any more.
I hadn't considered that the 1-1/8" bore master would take more pressure than the 1". But I do have a spare 1" I could try to see if it helps. Yes I am always bench bleeding the masters.
I went with the 9" booster for valve cover clearance.
Thinking about it more, if vacuum was my problem, shouldn't I be able to rev the engine while in park to a higher RPM and generate more vacuum for a few seconds, at least enough to "charge" the booster, then pump the brake once and I should have power assist?
This kit has the later 70's style "combination" valve, so no external block or hold off. I replaced the rear cylinders a few years ago but maybe I do have a bad or stuck one now. I'll check that also.
I haven't checked with MBM yet but when I called them on a different question a few months back and asked about that 18" vacuum requirement, they were pretty firm on that number. But at the time I didn't take it too seriously as I had heard others running much lower than that.
I was throwing my 2 cents in, I did the conversion too, I used the right stuff conversion kit and it came with a 9" booster for the same issue valve cover clearance. The kit had the same combo valve and it was causing my rear wheels to lock up if I put too much pressure on the pedal, always had to be gentle and ease it slowly. Thats when I went to a stock 1967 Firebird Distribution Block, front brake proportion Valve and the rear Hold off It stopped locking up. As far as Vacuum is concerned, I have a low vacuum cam. I run about 12-14" and I also have to wait for the booster to charge up, then it holds. Are you sure your booster isn't leaking? or Your Check valve is it working right? I have something on testing out brake boosters a friend sent me last year, I'll see if I can post it here.
I checked and adjusted pedal free play and rear drums.
Booster check valve I think is OK. I can't blow air in but can suck air out. As far as it holding pressure, I don't know. But I know it did work at one time and the only thing I did when it quit working was vacuum bleed the rear brakes.
Revving the engine to 2000-3000 rpm then pressing the brake pedal doesn't seem to have any affect. But my vacuum gage died so I don't know how much vacuum I'm getting.
Brake pedal still hard as a rock and doesn't stop very well.
As far as Vacuum is concerned, I have a low vacuum cam. I run about 12-14" and I also have to wait for the booster to charge up, then it holds. Are you sure your booster isn't leaking? or Your Check valve is it working right? I have something on testing out brake boosters a friend sent me last year, I'll see if I can post it here.
Ragu, what's your engine size, compression ratio, cam specs and current initial timing?
