NM'87 951

I've bled the system after installing a BMc and I'm having stiff pedal feel but no grab of the brakes. Not sure where to go from here.

Help appreciated.

william_b_noble

check that the calipers can actually compress the pads - you may want to pull a caliper and press the brake pedal and see the piston move

NM'87 951

I was told something similar. Get it on blocks and then see if I can spin any of the wheels with the brakes applied. Perhaps, if I get bad grip on a wheel after that test, I would pull the caliper and check it out?

bonus12

Exactly, the wheel should be un-turnable even with slight braking. If you can turn the wheel by hand while braking, your probably having a problem with your piston. It might be frozen. At this point your next step is to remove the caliper and see why the pads aren't being pushed against the disc. If the piston is free, there must be another reason why the fluid isn't pushing it.

As a quick test, have you tried really slamming on the brake (while stopped)? Try repetitively putting all your muscle into it. This could free the piston.

It just seems like the fluid isn't able to flow in the system.

NM'87 951

That's what my landlord said, like the car was having prostate problems. I did a few skid stops, while moving. Ill try the freeing piston technique with the car stopped. Then ill take the calipers off of suspect wheels after trying to spin them with the brakes applied.

Thanks much for the direction, ill update.

Arominus

New master cyl? is the push rod through the center of the booster in the correct place? when I did the master on my car it fell out and I thought it sat loosely in the booster, I was wrong. Got it together and the pedal was stiff but I had no brakes. Took it back apart and seated it in its grooves correctly, the push rod stayed in place and centered correct and then the brakes worked great.

plumbum

Are you getting power brakes? Might be something wrong with the booster?

When driving and you hit the brakes does the car pull? We're any calipers left hanging on the brake line or were the lines squeezed?

mytrplseven

Keep in mind, if you pull the calipers and push on the brake pedal, expect the piston to blow out of the caliper. Get a large pan under it because there'll be lots of brake fluid on the floor. And keep the pads out of it so they don't get soaked with brake fluid.

NM'87 951

I didn't think to worry about proper seating, as i thought it just slipped in and out so easily. I'll see about draining the reservoir and pulling off the BMC to check the seat of the piston inside the brake booster. Since I did no such alignment check when I put the bmc back on, this could be a culprit.

No pull, I left the calipers on when I did the bleeding. May be I'm confused by your question.


Thanks for the tip. If I gotta pull the calipers, I'll remember that.

John_AZ

NM'87

If you still cannot get a firm brake pedal you have an air pocket in the BMC.

Did you eventually do a bench bleed on the BMC?

https://rennlist.com/forums/924-931-...nt-w-pics.html

Do you have a firm clutch pedal?

J_AZ

plumbum

Air should cause a soft pedal.

Check the vacuum source to the booster would be my best guess..or as was mentioned the push rod is not seated properly

NM'87 951

Yea, the clutch is working as it was prior to doing the bmc. still catches in the same place, and still engages without difficulty through all the gears. I have a firm brake pedal, which catches high up in the range of travel, but it feels like it hits resistance and doesn't the brakes don't catch. I used a bit of muscle, and worked the pedal down hard (trying to free up any piston that might be stuck). I can get the brakes to grab better now, if I put some muscle into the brake pedal, but there's no grab when I just try to apply the brake in a consistent manner (if that makes sense).

I did not bleed the cylinder. My justification for that is two fold. 1. I stripped one of the hard line connectors, and had it repaired enough to finally get it screwed in and then fit the bmc in place. This process would have caused fluid to come out of the bmc all over the place, and air could have very easily gotten back in. 2. I assumed that running enough fluid through the cylinder while bleeding the brakes, with some light tapping would have freed up or burst any air bubbles in the bmc. I might be taking the bmc out to seat the piston, so may be I'll bleed the bmc @ the bmc (by removing the hard line connector to the rear bias valve and bleeding from there). Someone told me that works too.

The vacuum system is my next project. The vaccum system seems a mess, heater hose, repaired t connections... I've got the hose from lindsey, and will hopefully order a venturi delete soon and get the whole system re-hosed. I had assumed that if the vacuum system was at play, I wouldn't get any pedal resistance (or at least much less resistance) when I pressed on the brakes.

I know how to turn a wrench, and have a general road map of how a car works/operates. I have little to no experience with this stuff, so you guys are truly helpful. I appreciate it.

plumbum

The booster is vacuum assisted, if there is no vacuum getting to the master it will have no power assist breaking. There is also a one way check valve I believe at the booster

NM'87 951

I disconnected the T line that feeds the vacuum to the brake booster, for access to the bmc. I reconnected it afterwards, with no changes or anything.

Is there a step I am missing?

The initial symptoms (to remind everyone), was a pedal that would travel low and grab the brakes, but would then release the brakes as the pedal faded. We were told a leak @ the bmc, so we replaced it (we found the possible culprit in a leaky o-ring on the old bmc piston).

Shawns 83 944

I would suggest not removing the calipers just to see if you have a frozen caliper. What I would do is get a clear hose and attach it to the right rear bleeder valve and run it into a pan. Then have someone push the pedal down as you open the valve. If the pedal drops you have fluid travel to that caliper. If not go to the next caliper (left rear) and repeat, then the right front and left front. Just like a bleeding sequence, but this will tell you if it is in fact the calipers or there is a blocked line or master cylinder. If you get pedal travel through only one, two, or three lines, you may have a blocked line or frozen caliper or piston. The frozen piston isn't very common in any vehicle, its usually rust/corrosion on the caliper slide pins that makes a caliper stick. I would recommend cleaning and lubricating caliper slides whenever the pads are changed and always flush the fluid at least once a year on a daily driver and more frequently on cars that run high brake temps (racing etc etc)

I have also seen blockage in old hydraulic flexible hoses and on modern vehicles inside the check valves.

If your pedal does travel to the floor when all four bleeders are opened one at a time, then you more than likely have a brake booster problem. If the old master cylinder was leaking at the back seal, then the fluid more than likely damaged the booster. Sometimes at the brake shop I used to manage we could suck the fluid out depending on the type of booster, but its always better to replace with a new one.

Someone posted to make sure the pushrod is lined up correctly. I would definitely check that before going through the bleed sequence.

I try to keep this short, but there is so much information about brakes that can be helpful to someone. Trade knowledge is the best information a guy can have in automotive.