My neighbour asked me to do a post for him regarding the brake system on his 83 F-150 351 4x4. He has replaced the master cylinder for the third time in as many years trying to get a better pedal feel. He has re and re'd the whole system with the exception of the booster and the bias valve. The brake pedal goes nearly all the way to the floor before the brakes engage and it will barely stop the vehicle. The vaccuum is pulling only 12 psi at 1000ft above sea level. I would think that it should be at least 15. I checked the length of the push rod coming out of the booster and it is just under 1 inch, which is what the Chilton book calls for. One other thing that I noticed is that the vaccuum line from the disributor goes to a port on the carb that does not produce vaccuum at idle. Should the vac advance goe to manifold vaccuum? In doing a brake test where you bleed off booster vaccuum with the engine off and then start the engine with the brakes on, the pedal is hard and then goes down a bit which I always thought was normal. There are no vaccuum leaks and the brakes are all new and bled and rear brakes are set up properly. This is his only vehicle and would very much like to resolve this problem A.S.A.P as he cannot use it but for short trips and feels it is unsafe to use on the highways (for sure!) can any one give any input to this problem?
Hi, a good thing to do is get a power bleed done at a shop. I feel there is air in the system still. The booster vacuum should be connected to manifold vacuum not ported. the test you described on the master/booster sounds ok at this time. The manifold vacuum i would tend to agree should be 15mg. I hope this helps somehow.
Have the previous master cylinders leaked at all? If they did, they fluid could have worked it's way into the booster. If the booster is bad it would either make the pedal hard or you would hear a nasty hissing sound when applying the brakes. check the vacuum line to the booster. It may look OK but it may have some vibrated chaffing marks on the underside. These marks, If they are there, will not look like much but can actually cause enough of a leak to make the booster operate differant. Good luck. Jake.
You said " pedal goes almost to floor before the brakes engage ", which, leads me to beleive that the bias valve is stuck in the front only position. One trick I use while bleeding manually, always with the engine running, is to crack open a front bleeder and bleed both rear wheels twice before moving to the front brakes. The pressure lost with the front bleeder open will usually recenter the bias valve. I have also had to remove the safety switch and physically recenter the spool valve,bleed normally, and verify the valve did not go off center again. Good luck
How can the large vacuum line running from the booster go to a connection that has no vacuum at idle? All the ported vacuum connections are small vacuum line connections. The brake booster usually has a 3/8" line and is connected to manifold vacuum. This should definitely be investigated.
Hi all, the brake booster vac line went to a manifold tree. The question about the carb port was regarding the distributor. I comes from a port that doesn't have vaccuum at idle so I assume that it gets vaccuum when the throttle open causing the timing to change. The reason I asked this is that I thought the vaccuum level at idle was a bit low and incorrect timing can cause this but I need to know if the distributor gets vaccuum at idle or not as the books say to block off the line to the distributor with a golf tee before checking the timing and as it is now there is no vaccuum at idle so what would the need be for plugging it off? I want to jack the rear wheels up and try the brakes to see if they are actually working. When the master cylinder was changed at a reputable(!) shop by a certified mechanic he didn't bleed the back brakes because there was no sign of brake fluide around the bleeder and there was still old rust on them. I am not sure if the front ones were done either. He took the truck back to bitch about this so the shop owner did the brake bleed but did it with engine off and I felt that he should have bled them more than he did because the air in the line would be at the master cylinder end and it is a long way to the rear brakes to get the air out completely. It could be possible that the bias valve is off center as the bake brakes won't lock up at all and the truck does a nose dive when the brakes finally catch but you cannot get them to lock up and my neighbour is a BIG guy with size 16 feet!! I guess I will keep on trying to get them to work better for him.
Sorry about the vacuum line mix up. The distributor line should go to ported vacuum (no vacuum at idle). I believe the instruction to block the vacuum line is a generic "good practice" thing to follow when checking the timing on any vehicle and also guarantees if the idle is set a little high, there will be no vacuum on the dist. With a low pedal I would check and make sure the rear shoes are adjusted all the way out to the drums and there is no air in the system. After this I would be suspicious of the booster. I am still a firm believer in getting the master cylinder and booster as a set already setup at the factory. Advance used to sell them this way.
I replaced my master cylinder twice (Explorer) before I found that I was not properly bleeding the rear brakes. Same symptom.
I used the one-man bleeder system (vacuum pump), then the two man system, where one man presses the pedal for you. But I didn't get a good bleed until I again used the vacuum and pulled several ounces of fluid, then more air. I guess there was an air pocket, and when I first bled the lines I didn't get to it.
Of course, I followed instructions and bench-bled the master cylinder.
Anyway, that fixed it for me.
As a side note, when I built my 54 F100 with under the floor booster, I finally put an 87 F250 master cylinder on it to reduce pedal travel. With the larger bore cylinder and booster, braking is still easy and pedal travel is minimal.
There have been a couple of threads on (I believe) this site concerning the same problem. I've been having the problem for years now and am going to replace the calipers and flexible hoses next. That will be a complete replacement of everything except the hard lines over the last couple of years. My 1st attempt to repair the problem was replacing the complete master cyl/booster combination. That helped for a while until it went out. I replaced it again but still not a great deal of improvement. Next was the rear slave cyls, they were frozen solid when I checked them, apparently they hadn't been working for a long time but everything goes bad slowly so you don't notice. Why they were frozen is a mystery to me. I thought surely that would take care of it but it was not to be. Next was another master cyl then the proportional valve....nothing...absolutly no change. Finally, another master cyl just for grins ( the one I got after the last booster/cyl assembly was lifetime). Between the replacements I had the thing looked at by Ford and also by an independant, power bled and waved a dead chicken over it at midnight....not much better, but the chicken was good over the bar-b.
GOOD LUCK and I'm following along.
'86 F150 Super with a brand new used 5.8 installed.