Hi, I have a 1958 Impala I am almost finished with and I cant get the power brakes working. The pedal is hard as a rock. I have a front disc conversion on it, and a new booster and master cylinder combo that is designed for this setup on the car. I checked for vacuum and I had 16lbs so I bought a canister which jumper the vacuum up to 35 lbs which is plenty, still hard as a rock. I checked the front brakes to make sure they are free and they are. The front brakes are stopping but not really well, and the rears are not stopping at all. I do not have a proportioning valve so I am going to order one tonight. I figured without one id still have front brakes, Do you think this could cause the hard pedal? Ive tried adjusting the pushrod and doesn't change anything. And Also the booster is holding vacuum so the booster is ok. Also I disconnected the master from the booster to check the pedal for binding and it is free. Anyone with any ideas??? Thanks guys!
did you bench check master cylinder? did you contact company that you got the kit from? mpbrakes.com helps
I did not bench bleed the master. I hooked it up and bleed it out manually. All the air is out. Would this still cause a hard pedal?
First of all, 16 lbs of vacuum is tough to do since 14.7 psi of vacuum is the max you normally get..... So what are you using to measure vacuum? Posted using the Full Custom H.A.M.B. App!
No, you can get mid 20s"hg on decel. Somewhere around 18"hg at idle depending on cam and idle speed. Hard pedal all the time would be a booster only problem if above vacuum is present. When you disconnect booster with engine off do you get the WHOOOSH at the vacuum check valve?? In other words, the valve that goes directly into the booster. If you pull that out do you get the WHOLOOSH of air filling the booster chamber??
When I disconnect the check valve I do hear the "whoosh" so there is vacuum present. And I am using just a simple vacuum gauge hooked into the line after it goes into the vacuum canister. Maybe I read the gauge wrong but I don't think vacuum is the problem. And since the booster holds I assume that is ok as well. I saw in a leter post that a guy is having problems with his booster as well and yes it is a cpp 7" booster/ cylinder combo. Looks like they are not the company to go with next time but as for now Id like to figure out what to do. Any ideas? Also would not having a brake valve cause a hard pedal? Thanks
High hard pedal is normal on a vehicle with power brakes when booster is disconnected, bad or leaking. You have a high hard pedal right? With engine running, do you hear a vacuum leak at booster when booster is connected? On any other car I would be suspecting booster... The only reason I'm not sure is that I can't see the installation. Maybe something is binding or misaligned at m/c??
Just my opinion, but without a P valve, I would think the rears would lock before the fronts ever could. Just a question; how do you know the rears "aren't working"? By driving feel, or by hand turning, if the wheel is jacked up?
I jacked up the front and had someone push the pedal and the fronts stop but they can still be spun when you spin hard. I did the same thing to the rears and they free wheel. Why would the rears stop first? I just figured the fronts would stop first because the fluid has least amount fo travel to the calipers, but maybe the design of the caliper vs. wheel cylinder causes the rears first? I was hoping the rears were stopping the car because that would explain why it didn't stop well and then that would tell me a proportioning valve is needed but like I said the rears just free wheel.
Have you checked that you have the correct pushrod? There is no "travel time" with the fluid. Since the system is full, 1cc in is 1cc out, instantly. Post a picture of the master cylinder and please indicate where the front/rear lines are on the MC.
Without a P valve, the force on each wheel is by the ratio of square inches of the caliper piston and the rear wheel cyl piston, compared to the MC bore square inches. So, if you had unlimited sources of caliper and wheel cyl sizes and shoe contact areas, you could design a system that might stop equally, without a P valve....in threory. If your rears are not working at all? Something is way wrong, but what? If you don't mind clamping off the 2 front flex lines, you could test just the rears. If the rears still won't stop with fronts pinched; it could be a master problem like an internal valve backwards, or an undrilled orifice, stopping all flow to rear? This is a tough one over the net, for help. "If" it had a OEM P-valve with the saftey light switch, we'd all suggest that the switch valve piston is off to one side. That causes a blockage of flow, so that's why I was thinking a blockage at the MC at the rear port?
I tried loading a picture but it apparently it needs a website to load from so it's just a typical setup. The line towards the front of the car goes downward, then splits into both front wheels. The rear port of the master goes to the rear axle where it splits and goes to each wheel cylinder. I just saw the last reply and I will try clamping the front hoses tomorrow. Thanks for the ideas
Click on the "Go Advanced" button. In the "Advanced" window, you will see a little paperclip. That is how you attach a picture that is located on your computer.
The acid test for checking the booster is: With the car off, pump the pedal a few times to exhaust the booster. With your foot on the brake, start the car, the pedal should drop noticeably. Also if you hit the brakes and the engine starts to misfire, that's a sign of a bad booster. If you have a hard pedal, you don't have any air in the system. Another guess is, are the lines hooked up correctly at the master? Bob
Alright let's see if that worked. Now the front line closer to the front of the car goes down to a tee where it splits to the front wheels. The rear line goes to the rear axle where it splits to both wheel cylinders. Also when I evac the booster with the engine off then hold the pedal and start the car the pedal drops so the booster seems to be working right?
Pic works; it may not stop for beans, but the W motor looks cool as heck Do you have great flow at the rear bleeders, if someone holds the pedal down hard, and you open a bleeder a lot, very quickly? I still don't get why there is no rear brakes at all, if the pedal is hard. Trying to narrow it down.
So when you bleed the brakes, you do get easy and full pedal travel and plenty of fluid out of all the bleeders with relatively the same amount at all 4 wheels. If yes - We can say there is no mechanical binding in the pedal assembly We can also say the master is pushing fluid we can say that there is no obstruction to the fluid. At this point I would start looking for a mechanical bind of the brake components at the wheels. If no- There's some trouble between the brake pedal foot pad and the master. Un bolt the master from the booster and cycle the pedal, eliminate that end of this if you haven't already. With the master off place it back up to the booster, be sure pedal is fully up- the master should go on without the plunger of the master being contacted. That's all I got without seeing it.
IIRC the pushrod actuates a valve to help the master cylinder operate. My guess is that you have too long a push rod and you actuate the master cylinder way before the booster valve opens. Try a shorter push rod and see what happens.
Did your disc conversion, master cyl, and booster all come from the same mfgr as a complete system ? It sounds as if something is mismatched..... 4TTRUK
I had a tough time bleeding the rear brakes. The front had plenty of pressure but the rears did not. Tonight I am going to crack the lines at the master and check for pressure there when the pedal is applied. If that is ok I will check pressure at the wheels again.
The internal pushrod could be adjusted too long. That might explain having trouble bleeding the rears. When i install a MC to a booster, I hold the MC up against the booster and feel if the pushrod it hitting the MC piston. Easier to do with lines off, but with some patience, you should be able to tell, by removing the MC nuts and pull back on the MC.... Then see if you can feel it hit as you bring the MC closer to the booster I bleed brakes in a different way than I was taught. I open one bleeder 1/2 turn or so, then by hand, I slowly push the pedal down maybe 3/4 of the way, but then release it much slower, so the bleeder does not draw air back in. It always works for me. Once it is bled out and you can't hear air spitting, tighten and do the other side
Those little 6" power boosters are meant to run with a brake pedal ratio close to 7:1 Usually for underfloor mounting on hot rods, it appears you have the factory linkage set up in place and you have a pedal ratio closer to 4:1. If you want to keep this booster you may have to move the pushrod location closer to the pivot on your brake pedal to increase your leverage against the brake system.
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